What is ISSA (UK) 5000?
ISSA 5000 is the global baseline standard for sustainability assurance, and the FRC's UK version tailors it to the UK context alongside the rollout of UK SRS4.
What ISSA (UK) 5000 covers
The standard is a framework-neutral assurance standard: it can be applied to sustainability information prepared under different reporting frameworks, not just one2.
In broad terms it addresses:
- Engagement acceptance, ethics and independence requirements for the assurance provider
- Planning and risk assessment over the sustainability information
- Obtaining evidence — including over greenhouse-gas data and estimates
- Considering the reporting criteria (e.g. UK SRS) the information is prepared against
- Forming and expressing the assurance conclusion in an assurance report
Because it is framework-neutral, the same standard can support assurance of UK SRS S2 climate disclosures, CSRD/ESRS reporting, or voluntary frameworks2.
Limited vs reasonable assurance
ISSA 5000 supports two levels of assurance, and the distinction matters for cost and confidence2.
| Feature | Limited assurance | Reasonable assurance |
|---|---|---|
| Level of confidence | Moderate | High |
| Conclusion form | Negative (“nothing came to our attention”) | Positive (“in our opinion…”) |
| Work effort | Lower | Higher |
| Typical starting point | Early-stage sustainability reporting | Mature reporting / higher stakes |
Many sustainability regimes are expected to begin with limited assurance and move toward reasonable assurance over time as data and controls mature4.
How ISSA (UK) 5000 applies to UK SRS
ISSA (UK) 5000 is the assurance counterpart to the UK SRS reporting standards: UK SRS sets out what to disclose, and ISSA (UK) 5000 sets out how that disclosure is independently checked1.
UK SRS S2 is proposed to apply to listed companies from 1 January 2027 under DBT/FCA proposals5; the assurance expectation that sits alongside it is built on ISSA (UK) 50003.
For the wider picture of who must obtain assurance and when, see our sustainability assurance guide.
Who provides ISSA (UK) 5000 assurance
Assurance under ISSA (UK) 5000 is provided by auditors and other accredited assurance practitioners who meet the standard's ethics, independence and competence requirements2.
This includes accountancy firms and, where permitted, other professional service providers — a market the accountancy profession has been preparing for as UK SRS takes effect4.
What is ISSA (UK) 5000?
ISSA (UK) 5000 is the FRC’s UK sustainability assurance standard, adapted from the global IAASB ISSA 5000. It governs how assurance providers plan, perform and report a sustainability assurance engagement, including the assurance of UK SRS reports.
Who issued ISSA (UK) 5000?
The Financial Reporting Council (FRC) issued the UK version. It is based on ISSA 5000, the global sustainability assurance standard developed by the International Auditing and Assurance Standards Board (IAASB).
What is the difference between limited and reasonable assurance?
Limited assurance gives moderate confidence and a negatively worded conclusion with less work; reasonable assurance gives high confidence and a positive opinion with more work. Sustainability reporting often starts with limited assurance and moves toward reasonable assurance over time.
Does ISSA (UK) 5000 only apply to UK SRS?
No. It is framework-neutral and can be used to assure sustainability information prepared under different frameworks, including UK SRS, CSRD/ESRS and voluntary standards. In the UK its primary use is assuring UK SRS disclosures.
Is sustainability assurance mandatory in the UK?
Not initially. ISSA (UK) 5000 is intended for voluntary use, and FCA CP26/5 does not mandate assurance at the outset — though voluntary uptake is expected to rise. The standard is effective for engagement periods beginning on or after 15 December 2026, and is the standard engagements follow where assurance is obtained.
