Amended Company Tax Returns Online from April 2027 — UK Guide

HMRC plans to make online filing mandatory for amended Company Tax Returns from 1 April 2027. The proposal is part of the wider modernisation and standardisation of Corporation Tax submissions. It is aimed at reducing manual processing, improving data quality and speeding up amendments involving claims or repayments.

The planned change concerns how an amendment is submitted, not whether a company still has the legal right to correct a return. HMRC’s consultation says the existing amendment rights and statutory time limits are not being changed by this proposal. Companies should therefore prepare their software, records and approval process without treating the 1 April 2027 date as a new amendment deadline.

The proposal remains a policy and implementation item rather than a reason to invent final technical specifications. Follow the final HMRC service instructions, legislation and any published exceptions before submitting a real amendment.

What is changing?

At present, companies can generally amend a Company Tax Return through commercial software, while some amendments may be submitted by written correspondence. HMRC’s consultation proposes to close that channel for most in-scope amendments from 1 April 2027 and make electronic submission the default requirement.

HMRC’s stated aim is to reduce manual errors and delays, especially where an amendment includes a claim or repayment. A company still needs to establish the correct tax treatment before it files.

What does not change?

The planned online filing rule does not automatically:

• extend the time in which a company can amend its return;
• shorten the statutory amendment window;
• make an incorrect adjustment valid;
• remove the need to retain calculations and evidence;
• turn a tax enquiry into an ordinary amendment; or
• determine whether HMRC will accept a claim or repayment.

The existing GOV.UK guidance says companies usually have 12 months from the filing deadline to make changes, subject to the applicable rules and the type of claim. Use the deadline for the actual accounting period and return rather than assuming every correction has the same route.

That deadline is not the same as the deadline for the original return or the deadline for paying the tax. A Company Tax Return is normally due 12 months after the end of its accounting period, while Corporation Tax is usually due 9 months and 1 day after the period ends. The amendment clock normally runs from the return’s filing date or statutory filing deadline, so put the accounting-period dates, original filing date and amendment deadline in the same review record. If an accounting period was shortened, lengthened or split into two returns, calculate each return separately rather than applying one date to the accounts as a whole.

The receipt of an online amendment is also not an approval. HMRC can check the figures, open an enquiry or correct an obvious error or omission. Treat the submission acknowledgement as evidence that a notice was received, not as confirmation that a repayment is due or that a claim has been accepted. A revised self-assessment may change the amount shown as payable, but the company should reconcile that change to the HMRC account and separately confirm how any payment, interest or repayment will be dealt with.

Choosing the right correction route

Start by classifying the problem before changing the CT600. A simple omitted invoice, corrected estimate, arithmetic error or claim that belongs in the return may be dealt with by an amended return if the normal window is open. A change discovered after that window may need overpayment relief, a disclosure, an HMRC assessment or another statutory claim. The route depends on whether the company paid too much or too little, why the original return was wrong and whether the relevant relief has its own time limit or form.

If the correction increases the liability, tell HMRC promptly. Do not wait for an online amendment facility to become available if doing so could leave an inaccuracy unreported or allow interest and penalties to increase. If the correction reduces the liability, check that the company has a legal entitlement and that the revised computation supports the amount before requesting a repayment. An amended return is not a general-purpose request to reopen any historic accounting period.

The payment process is separate. Amending the return does not automatically cancel a payment instruction, recover a payment already made, or settle an overdue bill. Keep a separate schedule showing the original liability, payments, revised liability, interest and any repayment or set-off expected. This matters particularly where the amendment changes losses carried forward, group relief, quarterly instalment payments, research and development relief, capital allowances or a repayment claim.

What to send and what to retain

The company or agent should be able to explain the amendment in a reviewer-friendly order. Record the accounting period and CT600 version, then show the original figure, the corrected figure and the reason for the difference. Link each change to the accounts, ledger, contract, invoice, election, claim schedule or other evidence that supports it. If the change is a tax adjustment rather than an accounting correction, document the rule used and why it applies to this company and period.

For claims, retain evidence of the qualifying expenditure, ownership, dates, connected-party position and any required notification or report. For group relief, reconcile both the surrendering and claimant companies and check whether a joint amended return or amended supplementary attachment is required. For an R&D or creative-industry claim, use the dedicated HMRC conditions and submission requirements rather than assuming that a narrative letter can replace the prescribed information. For losses and capital allowances, show the effect on the current period and on later periods so that a later return does not continue to use an obsolete figure.

Agents should also confirm that they have the correct company authorisation, UTR, accounting-period dates and software profile. A technically valid computation can still fail if it is attached to the wrong period or submitted for the wrong company. Save the final output, submission timestamp, acknowledgement, error messages and any correspondence. If a filing is rejected close to the deadline, escalate promptly and retain evidence of the attempted submission; do not silently replace the rejected file with an unexplained figure.

A practical transition timetable

Before April 2027, list every open amendment window and flag cases likely to be submitted after the planned start date. Ask the software provider whether the product will support amended returns, revised computations, group-relief consequences and the relevant claim schedules. Build a controlled test using representative data, but do not submit a test return to HMRC unless the provider and company’s tax adviser have defined the process.

During the transition, monitor the final legislation, HMRC service guidance and the published exception list. The consultation proposed online filing from 1 April 2027, but it also asked for views on timing and exemptions. The implementation details therefore need to be checked at the point of filing. If the final rule differs from the consultation, the final rule controls this guide’s planning assumptions.

After the first live filing, compare the submitted CT600 and computation with the approved internal version. Confirm that the HMRC account reflects the revised liability and that any repayment or payment action has been completed. Record lessons about rejected fields, unavailable schedules and evidence requests so the next amendment is not prepared from memory.

Proposed exceptions and practical boundaries

The consultation describes exceptions where correspondence may remain available or the mandatory online requirement may not apply. The proposals include:

These are proposal-level boundaries. A company should not rely on an exception solely because a return is complex, a claim is large or the software is inconvenient. Confirm the final rule and retain evidence of any service outage or special status.

The 1 April 2027 date

The proposed start date is 1 April 2027 onwards. This is a filing-channel date, not a tax-year boundary. The relevant question will be when the amendment is submitted and which final rule applies, not simply whether the accounting period is labelled 2026 or 2027.

Companies with amendment rights expiring around the transition should plan early. If a return can be amended before 1 April 2027, the company should still use the correct current route and should not delay a valid correction merely to wait for an online service.

If the amendment window would expire during a documented HMRC service outage, follow the final contingency instructions. Do not treat an ordinary software problem or internal approval delay as an HMRC outage.

Computation-format preparation

The consultation also covers more standardised Corporation Tax computations and XBRL-tagged data. The final full prescription, build period and later pilot are separate implementation workstreams. They should not be presented as if every new format requirement is already mandatory for 1 April 2027.

For an amendment, prepare a file that ties together:

1. the originally submitted return and computation;
2. the exact line or schedule being changed;
3. the revised computation and the numerical bridge from the original;
4. the legal and factual reason for the adjustment;
5. supporting invoices, agreements, elections or claim evidence;
6. the effect on Corporation Tax, instalments, losses, credits or repayments; and
7. the internal approval and submission reference.

This file is useful regardless of the final online format. It also helps separate an amended-return correction from a disclosure, enquiry response or new claim that needs another process.

Special attention for claims and repayments

HMRC says the online filing change is intended to improve processing where amendments include claims or repayments. That does not mean an online submission will be accepted automatically.

Before submitting, verify:

• the amendment is within the applicable statutory window;
• the claim is available for the accounting period;
• the amount reconciles to the accounts and tax computation;
• any R&D, creative-industry, capital-allowance or loss claim uses the correct schedule;
• group-relief and joint-return rules are respected; and
• the company can explain why the original return was wrong or incomplete.

Keep the submission acknowledgement and reference. If the service rejects a return, record the error, the deadline risk and the route used to contact HMRC.

Preparation checklist for companies and agents

1. Inventory all accounting periods with open amendment windows.
2. Identify amendments likely to be filed near 1 April 2027.
3. Ask the software provider when online amended-return support will be available.
4. Test the amendment workflow using a non-production or controlled filing process.
5. Create a reconciliation template from original return to amended return.
6. Confirm which evidence supports each tax adjustment or repayment.
7. Establish an escalation route for an HMRC service outage.
8. Recheck the final legislation and HMRC guidance before the first submission after the change.

Frequently Asked Questions

Official sources

• HMRC: Making changes to Company Tax Returns
• HMRC: Company Tax Return obligations
• HMRC: Company Taxation Manual — amended returns
• HMRC: Modernising and standardising company tax returns consultation
• HMRC Agent Update — amended Company Tax Returns from 1 April 2027

For related context, read the UK Corporation Tax changes guide, the R&D credits and QIP thresholds guide and the HMRC tax investigations guide. Check the live GOV.UK Company Tax Return amendment guidance before submitting a correction.