ISSA (UK) 5000 is the FRC’s UK version of the international standard for assurance engagements on sustainability information, covering both limited and reasonable assurance. It was issued in November 2025 for voluntary use by assurance providers of any profession and is effective for sustainability information reported for periods beginning on or after 15 December 2026, with earlier use permitted. Limited assurance gives a negatively worded conclusion based on less work; reasonable assurance gives a positive opinion based on work closer to an audit.

What is ISSA (UK) 5000 and who issued it?

On 12 November 2025 the FRC issued ISSA (UK) 5000, General Requirements for Sustainability Assurance Engagements. The FRC describes it as a UK version of the IAASB’s global sustainability assurance standard, aligned with the international text, intended for voluntary use by UK assurance providers and designed to be profession agnostic, so it can be used by firms that are not audit firms.

Paragraphs 1 and 2 say the standard deals with assurance engagements on sustainability information, which is information about sustainability topics such as climate, labour practices or biodiversity, including risks, opportunities, metrics and targets. Under paragraph 8 it applies to all types of sustainability information however presented, and under paragraph 14 it is meant for entities of any size, applied in proportion to the engagement.

When does it apply and how does it relate to other standards?

Paragraph 15 makes the standard effective for assurance engagements on sustainability information reported for periods beginning on or after 15 December 2026, or as at a specific date on or after 15 December 2026. Earlier application is permitted.

  • ISAE (UK) 3000. Paragraph 11 says ISSA (UK) 5000 is an overarching standard covering every element of a sustainability engagement, so the practitioner does not also have to apply ISAE (UK) 3000.
  • The audit. Paragraph 12 says it does not cover sustainability information that must be included in the financial statements; the auditor deals with that under the ISAs. Paragraph 13 says that where sustainability information is published with audited accounts, the accounts are treated as other information for the sustainability engagement, the mirror image of the auditor’s position under ISA (UK) 720.
  • Attestation only. Paragraph 10 limits the standard to attestation engagements, where someone other than the practitioner, usually management, measures or evaluates the information against criteria.

Is sustainability assurance compulsory in the UK?

Not at present. In its response published on 30 January 2026, the Department for Business and Trade said that whether to require assurance over future UK SRS reporting will be considered as part of its wider work on modernising corporate reporting, and that the FCA will consider requirements for listed companies. The government will create a voluntary, profession agnostic oversight regime run by the FRC, under which sustainability assurance practitioners can opt in by registering, with a public register. The FRC has been asked to have an interim register operating well ahead of the reporting year starting 1 January 2027, with legislation to follow when Parliamentary time allows. The response did not settle whether any future requirement would be for limited or reasonable assurance.

What is the difference between limited and reasonable assurance?

Paragraph 18 defines both, and paragraph 9 says every requirement applies to both unless stated otherwise. Where they differ, the standard prints the requirements side by side with L and R suffixes.

Feature Limited assurance Reasonable assurance
Engagement risk Reduced to a level acceptable in the circumstances, but greater than for reasonable assurance Reduced to an acceptably low level
Risk assessment (103L and 103R) Risks identified and assessed at the disclosure level Risks identified and assessed at the assertion level for the disclosures
Internal control (113L and 113R) Understanding obtained through inquiry Understanding obtained through inquiry and other procedures, with evaluation of the control environment and risk process
Conclusion (198L and 198R) Nothing has come to our attention that causes us to believe the information is not prepared, in all material respects, in accordance with the criteria In our opinion, the information is prepared, in all material respects, in accordance with the criteria

Paragraph 9 says that because the level of assurance in a limited engagement is substantially lower, the procedures vary in nature and timing and are less in extent. The definition still requires a meaningful level of assurance: one likely to enhance users’ confidence to a degree that is clearly more than inconsequential.

What does the practitioner examine?

  • The criteria. Under paragraph 3 the practitioner must evaluate whether the criteria are suitable. Framework criteria embodied in law or issued by recognised bodies following a transparent due process are presumed suitable, absent indications to the contrary.
  • How the entity chose what to report. Paragraph 4 refers to the entity’s process for identifying which sustainability topics to report and the reporting boundary, including how it applied materiality.
  • The scope. Paragraph 5 says the engagement may cover all of the sustainability information or only part of it, for example only climate disclosures, and the report must identify what was covered.
  • Legal framework and fraud. Paragraph 111 requires an understanding of the legal and regulatory framework and how the entity complies; paragraph 112 requires inquiries about fraud and non compliance affecting the information.

Who can perform the work and to what standard?

Because the standard is profession agnostic, it relies on equivalent safeguards rather than membership of an accountancy body. Paragraph 34 requires compliance with the IESBA Code provisions for sustainability assurance, plus stricter national rules, or requirements an appropriate authority has found at least as demanding. Paragraph 6 assumes the practitioner’s firm is subject to ISQM (UK) 1 or equally demanding quality management requirements. Paragraph 33 requires the engagement leader to have assurance skills developed through extensive training and practical application, an understanding of the ethical and independence rules, and enough sustainability competence to take responsibility for the conclusion. Independence for non audit assurance is discussed in independence in non audit assurance.

What if the practitioner cannot reach a conclusion?

Paragraph 17 says that where limited or reasonable assurance cannot be obtained and a qualified conclusion would not be enough to inform users, the practitioner must disclaim a conclusion or withdraw from the engagement where the law allows. Paragraph 199 provides for Emphasis of Matter and Other Matter paragraphs, as in an audit report.

How might this look in practice?

Illustration only. A listed retailer publishes Scope 1 and Scope 2 greenhouse gas emissions and asks a firm for limited assurance on those figures only. The practitioner checks that the reporting criteria are suitable, understands how store and warehouse energy data is collected, assesses risks for each disclosure, then performs analytical procedures and tests a sample of meter readings and supplier invoices. No issues are found, so the report states that nothing has come to the practitioner’s attention suggesting the emissions are not prepared, in all material respects, in accordance with the criteria, and makes clear that other sustainability content in the annual report was not covered. The company’s energy and carbon reporting data feeds directly into this work.

What are the common misunderstandings?

  • Treating a limited assurance conclusion as if it were an audit opinion. The wording is deliberately negative and the work is narrower.
  • Assuming the whole sustainability report is covered when the engagement covers selected metrics.
  • Believing assurance is already mandatory for UK SRS reporting; at present it is voluntary.
  • Assuming only audit firms may use the standard; it is designed to be profession agnostic.
  • Overlooking the entity’s own process for deciding what to report, which the practitioner must consider.

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Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 19 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.