Key takeaway: From 1 April 2026, the initial penalty for a late corporation tax return (CT600) has doubled from £100 to £200. Further penalties apply for continued late filing: a £200 charge at three months, tax-geared penalties at 18 months if the return is still outstanding. Combined with an already strict payment deadline regime and rising HMRC compliance activity, the cost of missed CT deadlines has increased meaningfully. For practices managing dozens of company clients, preventing late CT filings requires a systematic deadline management workflow — not a manual diary.
The doubling of the initial corporation tax late filing penalty is a quiet but significant change. At £100, the previous penalty was often treated by both clients and practices as an acceptable cost of occasional administrative slippage. At £200, it remains below the level that feels catastrophic — but the direction of travel is clear: HMRC is increasing the financial consequences of non-compliance across all its penalty regimes. The corporation tax change is part of the same trajectory as the MTD ITSA points-based system and the VAT late filing regime.
More materially, it is the reputational consequence of a missed CT filing that typically concerns practices most. A client who receives a penalty notice — even for a relatively small amount — will question whether their accountant is on top of their affairs. Preventing that conversation from happening is worth more than the penalty itself.
The Full CT Penalty Structure from April 2026
The corporation tax late filing penalty structure applies to returns not filed by the 12-month deadline (one year from the end of the accounting period). From 1 April 2026:
Day one to three months late: an initial penalty of £200 (previously £100), charged immediately the deadline is missed.
Three months late: a further £200 penalty, giving a total of £400 for returns three months or more late.
18 months late: if the return is still outstanding at 18 months after the end of the accounting period (six months past the filing deadline), a tax-geared penalty applies — 10% of the unpaid corporation tax. For a client with a £50,000 CT liability, this is a £5,000 additional charge.
Repeated late filing: for companies that file late in consecutive years, the initial penalty increases to £500 at day one and a further £500 at three months — making repeated failure significantly more expensive than a one-off late filing.
The penalty structure applies regardless of whether the corporation tax itself has been paid on time. A company that pays its tax correctly but files the return late still incurs the filing penalty.
Late Payment Penalties and Interest: The Full Cost Picture
Separate from the late filing penalties, HMRC charges late payment penalties and interest on corporation tax paid after the due date. The payment deadline is nine months and one day after the end of the accounting period — three months earlier than the filing deadline. Companies that pay late face:
Late payment interest: charged at 7.75% per annum from January 2026 on unpaid corporation tax from the payment due date until payment.
Large company quarterly instalments: companies with annual profits above £1.5 million pay corporation tax in quarterly instalments during the accounting year. Late instalments carry interest charges that can be material for larger clients.
The combined impact of a late-filed return and late-paid tax — filing penalty, late payment penalty, and interest — can quickly exceed what the client or practice imagined when a deadline was allowed to slip.
Why CT Deadlines Are Particularly Vulnerable
Corporation tax filing deadlines are vulnerable to slippage for structural reasons that practice owners understand well. The CT600 cannot be finalised until the statutory accounts are complete. The statutory accounts depend on year-end adjustments, director and shareholder sign-off, and sometimes third-party information. Where the accounts run late — as they often do, particularly for clients whose records arrive late or where audit or review work takes longer than planned — the CT return timeline is compressed.
For practices managing a large portfolio of company clients, the concentration of December year ends (the most common accounting year end) creates a predictable workflow bottleneck in the first quarter of each year. Returns due by 31 December 2026 (for December 2025 year ends) all have a filing deadline of the same date. Without systematic visibility of which clients are on track and which are at risk, the late filing penalties from April 2026 will affect a number of clients every year.
How BrightManager Prevents Late CT Filings
BrightManager is Bright’s practice management platform. It tracks every client engagement, including corporation tax return filings, against the correct deadline — automatically calculating the 12-month filing deadline and the 9-month-and-one-day payment deadline for each company based on its accounting period.
Rather than relying on individual staff members to track CT deadlines through personal calendars or email reminders, BrightManager creates a centralised deadline register visible to the whole practice. As each CT return’s filing deadline approaches, BrightManager generates tasks and automated reminders — prompting the relevant team member to confirm that the return has been prepared and filed, or escalating to a manager if the return is at risk of being late.
For practices with large portfolios of company clients, BrightManager‘s dashboard view of upcoming deadlines allows partners and managers to see at a glance which clients have CT returns due in the next 30, 60, or 90 days, and which are progressing through the workflow on schedule. Spotting a return that is behind schedule at 60 days out allows the practice to take action and avoid the penalty. Spotting it at five days out is too late.
The integration between BrightTax (where the CT return is prepared) and BrightManager (where the deadline is tracked) means the workflow status is visible throughout the process — from accounts production through CT computation to filing confirmation — in a single platform.
Practical Steps for Practices in the First Year of the Doubled Penalty
Audit your current deadline management process. How does your practice currently track CT filing deadlines? If the answer involves individual team members’ calendars, spreadsheets, or memory, the risk of missing a filing as the portfolio grows is significant. The doubled penalty makes the case for a systematic solution more urgent.
Identify December year-end clients early. December year-end CT returns are due by 31 December of the following year. The bottleneck created by many returns falling due simultaneously is manageable with six to nine months of planning; it is very difficult to manage in the final six to eight weeks before the deadline.
Communicate payment deadlines to clients. Many clients conflate the filing deadline with the payment deadline. The payment deadline — nine months and one day after the accounting period — is three months earlier than the filing deadline for most companies. Clients who plan their cash flow around the filing deadline are routinely late payers. A proactive reminder from the practice before the payment deadline is good client service and reduces the interest cost.
Frequently Asked Questions
What is the corporation tax late filing penalty from April 2026?
From 1 April 2026, the initial penalty for a corporation tax return filed after the 12-month deadline is £200 (doubled from £100). A further £200 applies if the return is still outstanding at three months late. For returns outstanding at 18 months from the accounting period end, a tax-geared penalty of 10% of unpaid tax applies. Repeated late filing (in consecutive years) increases the initial penalty to £500.
When is a corporation tax return due?
The CT600 must be filed within 12 months of the end of the company’s accounting period. For a company with a 31 December 2025 year end, the filing deadline is 31 December 2026. The corporation tax payment deadline is earlier — nine months and one day after the accounting period end, so 1 October 2026 for the same company.
Does the late filing penalty apply even if the corporation tax has been paid on time?
Yes. The late filing penalty relates specifically to the failure to file the CT600 within the 12-month deadline, and is charged regardless of whether the tax itself has been paid. A company that pays its full tax liability on time but files the return late will still incur the filing penalty.
What happens if a company files late in consecutive years?
If a company files its CT600 late in two consecutive accounting periods, the initial penalty increases to £500 (from £200), and a further £500 applies at three months. Repeated late filing is therefore significantly more expensive than a single late filing, and HMRC’s records will flag a pattern of lateness.
How does BrightManager help practices prevent late CT filings?
BrightManager tracks CT filing and payment deadlines for every company client, generates automated task reminders as deadlines approach, and provides a centralised dashboard showing which clients are on track and which are at risk. This gives practices the visibility to act before deadlines are missed, rather than responding after a penalty has been issued.
BrightManager tracks corporation tax filing and payment deadlines across all company clients, with automated reminders and a deadline dashboard that gives practices the visibility to prevent the late filings that now carry a doubled penalty. Find out more about BrightManager or speak to your account manager.
