Found an old R&D tax relief error? What small companies should do next

Small companies that discover an old Research and Development (R&D) tax relief claim was too high have a specific HMRC disclosure route available when the normal deadline for amending the Company Tax Return has passed.

HMRC updated its guidance on 14 August 2026. For directors reviewing historic claims, the important point is that waiting for HMRC to find an error can increase the exposure to interest and penalties. The disclosure service gives eligible companies a structured way to calculate the amount due, explain the mistake and make an offer to settle it.

When the R&D disclosure service applies

According to HMRC, the service is intended for a company that claimed too much R&D tax relief, can no longer amend the relevant Company Tax Return because the amendment window has closed, and must pay additional Corporation Tax or repay an R&D tax credit.

A director or company secretary can make the disclosure, as can a tax adviser acting for the business. An adviser who is not already authorised may need to upload a COMP1a temporary-authority form.

The route is not suitable in every case. A company still within the amendment deadline should correct its return through the usual process. HMRC also says deliberate inaccuracies should be handled through the Contractual Disclosure Facility, not this service. Where an error only overstated losses and there is no tax or credit to repay, the guidance directs companies to contact HMRC’s R&D team instead.

What a small company needs to prepare

The online form asks for the company’s Unique Taxpayer Reference, registered address, SIC code, affected accounting periods, details of the original claim preparer where known, and an explanation of how the inaccuracy arose.

The calculation work is more substantial. A disclosure should include the original and revised Corporation Tax computations for each affected period, a breakdown of the original and corrected qualifying R&D expenditure, and workings for tax, credits, interest and any penalties due. HMRC accepts supporting files including PDF, spreadsheet and common document formats, subject to a 10MB limit.

Businesses should assemble those figures before opening the form because the service can time out. Progress can be saved for 28 days and refreshed by logging in and saving again, but HMRC says the form must be submitted within 90 days of starting it.

How far back should the review go?

The look-back period depends on how the error happened. HMRC’s guidance says disclosures should cover up to four years from the end of the relevant tax period where the company took reasonable care, or up to six years where it was careless. Deliberate behaviour belongs in a different disclosure process.

The distinction matters because it can affect both the number of accounting periods involved and the potential penalty. HMRC says penalties are usually lower for a voluntary disclosure, although the final position depends on the circumstances and the quality and timing of the disclosure.

What happens after submission

HMRC normally issues a payment reference number within 15 calendar days. It then aims to accept the offer, request more information or reject it within 30 calendar days. A company unable to pay the full amount can ask for more time through the service; HMRC says it will usually expect payment within 12 months.

Interest generally runs until the tax is paid, so identifying an error is a prompt to act, not simply to add it to the next year-end checklist. Directors may also want to revisit who approved the original claim, what evidence supported each cost category and whether the same control weakness affected other periods.

Practical next steps for SME directors

Confirm whether the Company Tax Return can still be amended normally.
Identify every affected accounting period and preserve the underlying R&D records.
Rebuild the full Corporation Tax and R&D computations, rather than calculating only a headline repayment.
Document whether the error arose despite reasonable care or through carelessness.
Consider independent professional help where the calculations, penalties or behaviour assessment are uncertain.

Companies should use the current official guidance rather than relying on an old checklist. BritishSME has also covered changes to HMRC’s Corporation Tax online service, another reason to check filing processes rather than letting them run on autopilot.