Sustainability assurance — FRC register and ISSA (UK) 5000
Independent verification of UK SRS disclosures is governed by two new instruments: the FRC Interim Register (operational mid-2026) and ISSA (UK) 5000 (effective 15 December 2026). Most UK listed groups start at limited assurance. Under FCA CP26/5, companies must disclose whether third-party assurance was obtained — mandatory assurance follows as the market matures.
Sustainability assurance — key figures
What the UK assurance framework looks like today: the FRC Interim Register, ISSA (UK) 5000, the applicable assurance level, and the FCA disclosure requirement.
A new sustainability assurance regime is taking shape across the UK regulatory landscape. The government confirmed in January 2026 that it will proceed with a voluntary, profession-agnostic oversight regime for sustainability assurance practitioners, with the FRC responsible for implementation and oversight. The GOV.UK assurance response confirmed the FRC has been tasked to establish the interim register by mid-2026 — well ahead of the 1 January 2027 reporting year.
Simultaneously, the IAASB published ISSA 5000 — the first global standard specifically designed for sustainability assurance — in November 2024, following approval in September 2024. The UK adaptation, ISSA (UK) 5000, is effective for assurance engagements on sustainability information reported for periods beginning on or after 15 December 2026. Companies preparing for UK SRS compliance should treat audit-readiness as an immediate operational priority, not a 2026 task.
The FRC Interim Sustainability Assurance Register
Temporary practitioner oversight while the permanent regulatory framework develops — competence, independence and quality standards for sustainability assurance work.
The FRC Interim Register is a voluntary, non-legislative public register of sustainability assurance practitioners. The government confirmed it will proceed following strong stakeholder support in the consultation response published on 30 January 2026. Statutory underpinning will follow when Parliamentary time allows, but the interim non-legislative regime proceeds regardless.
The register is designed to initially focus on sustainability assurance firms rather than sole practitioners, given the size and nature of entities within scope of climate and sustainability-related reporting requirements. Registration is voluntary — inclusion is not a precondition for performing engagements under ISSA (UK) 5000 — but it is expected to become a market-standard quality marker that procurement teams and audit committees will increasingly demand.
For companies selecting an assurance provider ahead of the register launch, the FRC register will provide a defensible quality marker — particularly important when choosing a non-audit firm. Organisations building the internal readiness to support an assurance engagement should consult the UK SRS readiness assessment framework, and ensure their carbon reporting systems produce audit-ready documentation with comprehensive methodology tracking.
ISSA (UK) 5000 and ISAE 3000 (Revised)
Two standards anchor sustainability assurance in the UK: ISSA (UK) 5000 as the dedicated sustainability assurance standard, and ISAE 3000 (Revised) as the international fallback for non-historical-financial engagements.
ISSA 5000 was approved by the IAASB in September 2024 and formally published in November 2024, following certification by the Public Interest Oversight Board. It represents a landmark shift: where ISAE 3000 applied broadly to all non-historical-financial assurance, ISSA 5000 is purpose-built for sustainability information, with 212 requirements — more than double those of ISAE 3000.
The FRC's UK adaptation introduces a single modification: ISSA (UK) 5000 prohibits the use of direct assistance by internal auditors in sustainability assurance engagements, consistent with the existing prohibition for financial statement audits in the UK. The standard is principles-based, framework-neutral, and designed to be usable by both accounting and non-accounting professionals — provided they meet the relevant quality management and ethical requirements.
- Limited assuranceStarting level
- Lower assurance threshold — the practitioner reports whether anything came to their attention indicating misstatement. Based on analytical reviews, inquiries, and walkthroughs. Most UK listed groups start here for sustainability reporting.
- Reasonable assuranceHigher level
- Closer to a financial-statement audit — the practitioner concludes positively that the reported information is, in all material respects, in accordance with the criteria. Requires comprehensive verification and detailed testing. Expected to follow as the market matures.
- ISSA (UK) 5000Sustainability-specific
- International Standard on Sustainability Assurance (UK) 5000 — the dedicated sustainability assurance standard adapted for the UK market, effective for periods beginning on or after 15 December 2026. Profession-agnostic, principles-based, framework-neutral.
- ISAE 3000 (Revised)International fallback
- International standard on assurance engagements other than audits or reviews of historical financial information. Continues to apply for GHG-specific engagements alongside ISAE 3410, but ISSA 5000 supersedes it as the primary sustainability assurance standard.
Requirements in ISSA 5000
ISSA 5000 contains 212 requirements — more than double those of the previous ISAE 3000 standard — providing more specific requirements to assure sustainability information and reflecting the unique complexity of non-financial data.
Sustainability assurance — frequently asked
Who can assure, what level, what evidence is expected, how to plan the engagement, and the bridge to financial-statement audit.
The questions below cover the most common practical concerns for finance directors, audit committee chairs, and sustainability leads preparing for UK SRS reporting. For the full regulatory backdrop, the FCA CP26/5 consultation paper (published January 2026) and the GOV.UK assurance consultation response are the primary reference documents.
What is the FRC Interim Register for sustainability assurance practitioners?
The FRC Interim Register is a voluntary, non-legislative registration system for sustainability assurance practitioners, targeted to be operational by mid-2026, ahead of the 1 January 2027 reporting year.
Registration criteria will include demonstrated competency in sustainability reporting, understanding of UK SRS requirements, and relevant professional qualifications.
The register initially focuses on firms rather than sole practitioners, given the scale and complexity of in-scope entities.
What is the difference between limited and reasonable assurance for sustainability reporting?
Limited assurance provides negative assurance ('nothing has come to our attention') based on analytical reviews, inquiries, and walkthroughs.
Reasonable assurance provides positive assurance requiring comprehensive verification and detailed testing, closer to a financial statement audit.
UK SRS initially requires limited assurance with potential progression to reasonable assurance based on market readiness.
Which emissions scopes require assurance under UK SRS?
Scope 1 and 2 GHG emissions are the primary focus for initial assurance under UK SRS.
Scope 3 emissions are subject to comply-or-explain provisions, and forward-looking disclosures such as scenario analysis may be excluded from assurance scope initially.
FCA CP26/5 requires listed companies to disclose whether third-party assurance has been obtained, the assurance level, and the standard applied.
What qualifications do sustainability assurance practitioners need?
ISSA (UK) 5000 is profession-agnostic — it applies to both professional accountants and non-accountant assurance practitioners, provided they meet the relevant quality management and ethical requirements.
Practitioners must meet ISQM 1 quality management standards and IESBA Code ethical requirements.
Core competency areas include climate science understanding, GHG accounting protocols, and sustainability reporting frameworks.
How does sustainability assurance differ from financial audit?
Sustainability assurance requires specialised competency beyond traditional financial audit experience, including climate science understanding and sustainability frameworks.
ISSA 5000 contains 212 requirements — more than double those of the equivalent ISAE 3000 standard.
Independence requirements are enhanced, and the FRC has prohibited the use of direct assistance by internal auditors in sustainability assurance engagements, consistent with the existing prohibition for financial statement audits.
Who can provide sustainability assurance?
ISSA (UK) 5000 is deliberately profession-agnostic — but quality management, ethical requirements, and FRC register status will become the practical differentiators in the market.
UK statutory audit firms — those eligible to audit UK listed companies under the Companies Act — are automatically eligible to perform sustainability assurance under ISSA (UK) 5000. They meet the FRC competence and quality control standards through existing registration and oversight mechanisms. Most large UK statutory audit firms have established dedicated sustainability assurance practices in anticipation of mandatory and voluntary demand.
Non-audit firms — including specialist sustainability assurance providers, management consultancies, and environmental data specialists — may also provide assurance under ISSA (UK) 5000, provided they meet quality management and ethical requirements equivalent to ISQM 1 and the IESBA Code. For these providers, registration on the FRC Interim Register will serve as the primary market signal of readiness. The regime is intentionally not designed to capture smaller or bespoke sustainability assurance service providers who do not provide assurance opinions at that level — though such providers are not excluded from registration if they meet the conditions.
- Big Four and statutory audit firmsAuto-eligible
- UK statutory audit firms are automatically eligible under ISSA (UK) 5000 by virtue of existing FRC registration and oversight. Most have established dedicated sustainability assurance teams ahead of 2027.
- Specialist sustainability assurance firmsRegister-eligible
- Non-audit firms with sustainability data expertise may register on the FRC Interim Register. They must demonstrate quality management standards equivalent to ISQM 1 and IESBA Code ethical requirements.
- Combined assuranceGovernance model
- An integrated approach where internal audit, risk management, and external assurance work in coordinated layers. ISSA (UK) 5000 explicitly prohibits external practitioners from relying on direct assistance by internal auditors, preserving the independence of the external engagement.
- Audit committee oversightBoard responsibility
- Audit committees of in-scope listed companies are expected to oversee the appointment of sustainability assurance practitioners and assess the scope and quality of the engagement, in the same way they oversee the external financial statement audit.
Assurance scope — what is and isn’t covered
Initial assurance covers Scope 1 and 2 emissions. Scope 3, forward-looking disclosures, and scenario analysis are subject to transitional relief and evolving FCA guidance.
Under FCA CP26/5 §7.6, listed companies must specify in their annual financial report whether they have obtained third-party sustainability assurance over their UK SRS disclosures — covering both UK SRS S2 (climate) and UK SRS S1 (general sustainability). Where assurance has been obtained, companies must disclose the assurance level (limited or reasonable), the assurance standard applied, the name of the provider, and where the assurance report can be located.
Forward-looking disclosures — including climate scenario analysis and transition plan projections — present particular assurance challenges and may initially be excluded from the scope of an assurance engagement. The IAASB ISSA 5000 framework acknowledges that the scope of an assurance engagement may extend to all sustainability information or only part of it, giving companies and practitioners flexibility to agree a proportionate scope. For guidance on Scope 3 reporting under UK SRS, see the dedicated reference page.
- Scope 1 and 2 emissionsIn scope — initial focus
- Direct emissions (Scope 1) and purchased-energy emissions (Scope 2) are the primary focus for initial sustainability assurance under UK SRS. These are the most measurable and verifiable categories.
- Scope 3 emissionsComply-or-explain
- Value-chain emissions are subject to transitional comply-or-explain relief under UK SRS. Where a company reports Scope 3, that information may be in scope for assurance — but exclusion is permissible with explanation during the transitional period.
- Scenario analysis and forward-looking disclosuresInitially excluded
- Climate scenario analysis, transition plan projections, and other forward-looking information present unique assurance challenges. Initial assurance engagements will commonly exclude these elements, with scope evolving as standards and practitioner capability mature.
- Evolving scope — FCA guidanceTo be confirmed
- The FCA CP26/5 Policy Statement (expected autumn 2026) will clarify the precise scope of required disclosures and any future mandatory assurance framework. The FCA has noted the possibility of mandatory assurance 'in due course' pending market readiness assessment.
Preparing for assurance — practical steps
Assurance-readiness is an operational task that starts now, not in 2026. Four steps companies should take ahead of the FRC register launch and first reporting year.
With the FRC Interim Register targeted for mid-2026 and ISSA (UK) 5000 effective from December 2026, the window for preparation is narrowing. The GOV.UK consultation response anticipates increased demand for assurance services even where voluntary — meaning well-prepared organisations will have a wider choice of providers and more favourable engagement terms.
Ensuring your sustainability data systems produce audit-ready output is the most important single step most companies can take now. The right carbon reporting software should maintain a full audit trail from activity data to reported figures, support multiple emission factor datasets, and export methodology documentation suitable for practitioner review. For a broader view of how assurance fits into the overall compliance programme, the UK SRS compliance guide covers governance, data, and reporting obligations in full.
Companies subject to FCA CP26/5 should note that even where assurance is not obtained, the obligation to state that fact — and to identify the assurance provider, standard, and level where assurance is obtained — creates a reputational dynamic. Investors and proxy advisers are likely to treat the absence of assurance as an indicator of reporting maturity. Early voluntary engagement with a registered practitioner signals organisational readiness of a kind that markets increasingly reward. See the UK SRS readiness assessment for a structured gap-analysis framework.