BRSR Core assurance in FY 2025-26: a practical guide for top-500 companies

FY 2025-26 is the second year that reasonable assurance on BRSR Core KPIs is mandatory for the top 500 listed companies. If you are advising any of these companies, or are being asked to prepare them for their assurance engagement, this guide covers what the assurer will actually check and how to build the right evidence trail from day one of data collection.

What BRSR Core actually is

BRSR Core is a sub-set of 42 KPIs drawn from the full BRSR format. These 42 KPIs map to 9 broad attribute areas that SEBI considers the most material and quantifiable: GHG intensity, energy intensity, water intensity, waste generated, complaints on essential indicators, openness of business, gender-wise pay ratio, inclusive development, and CSR spend. BRSR Core is not a separate report. Companies file their standard BRSR as part of the Annual Report, and the 42 Core KPIs within that filing are independently assured.

Reasonable assurance under ISAE 3000

Reasonable assurance is the same level of confidence as a statutory financial audit. The assurer must obtain sufficient appropriate evidence to express a positive opinion: "In our opinion, the 42 Core KPIs are presented fairly and free from material misstatement." This contrasts with limited assurance (a negative opinion: "nothing came to our attention to suggest material misstatement"), which requires substantially less evidence.

The ISF sector-specific standards

The Industry Standards Forum (ISF), a body constituted under SEBI's aegis, published sector-specific BRSR Core Standards in December 2024. These standards provide binding guidance on how each of the 42 KPIs should be measured, sector by sector. They are the authoritative reference for assurers reviewing BRSR Core data, and your measurement methodology should align with them. The standards are available on the SEBI website and are worth reviewing before your client's data collection begins.

SEBI March 2025: assessment as an alternative pathway

SEBI's March 28, 2025 circular introduced "assessment" as a lighter alternative pathway alongside full ISAE 3000 reasonable assurance. Assessment is an evaluation under the sector-specific ISF standards, conducted by an independent evaluator, but is less onerous than a full reasonable assurance engagement. The criteria for which companies can choose assessment versus which must complete full assurance are specified in the circular and depend on factors such as market-cap rank and filing history. If your client is considering the assessment pathway, read the March 2025 circular directly. Do not rely on secondary interpretations, including this post, to make that determination.

Five preparation steps that determine whether assurance goes smoothly

• Assign a named data owner per KPI. The assurer will ask who is responsible for each figure. Without a named owner, the evidence trail goes cold.
• Document your calculation basis. For every GHG, energy, and water figure, write down the formula used, the source data, and the emission or conversion factor with version number.
• Collect evidence at site level before aggregation. If the company has three plants, collect electricity bills for each plant separately, then sum them. Do not start from an aggregated number.
• Keep the original source documents. Fuel purchase invoices, electricity bills, sub-meter readings, HR system exports. These are the assurer's primary source material.
• Appoint the assurer early. SEBI-registered ESG assurance providers have limited capacity and are booking up months in advance as the Core mandate expands to the top 1000 from FY 2026-27.

Common failure modes

The most common reason for a qualified assurance opinion on BRSR Core KPIs is not a factual error in the reported number; it is a missing evidence trail. Data that was estimated, extrapolated, or back-calculated without a documented methodology is very difficult to assure to a reasonable level. Second most common: using data that cannot be traced to a primary source (for example, a number pulled from a management report rather than the underlying HR system export or utility bill).

Key takeaways

• BRSR Core covers 42 KPIs across 9 attribute areas. Companies do not file a separate report; the KPIs are within the standard BRSR Annual Report filing.
• Reasonable assurance under ISAE 3000 is the default standard for FY 2025-26 for the top 500.
• SEBI March 2025 introduced an assessment pathway as an alternative. Read the circular to determine which pathway applies to your client.
• ISF sector-specific standards (December 2024) govern how each KPI is measured. Align your methodology to them before data collection starts.
• The most common assurance failure is a missing evidence trail, not a wrong number. Build the trail from day one.