Several overdue CT600 returns: which should you file first?

In short: Usually the oldest first, because losses and balance sheet figures carry from one year into the next. Each late Company Tax Return (CT600) has its own penalties, starting at £200 for a return due on or after 1 April 2026 (GOV.UK: penalties for late filing), so deal with a later return sooner when an HMRC determination or a claim deadline makes it urgent. Taxley files returns back to accounting periods that started on 1 April 2020, at £44.50 (promotion price until 31 Dec 2026; £89.00 from 1 Jan 2027) each with micro-entity accounts, and files each year's accounts at Companies House once that return is paid and finished.

Take the 30-second check to see whether Taxley fits your company, or work out each year's penalties with the free Corporation Tax late filing penalty calculator.

Key facts (checked on 1 October 2026)

Which overdue CT600 should you file first?

Start with the oldest return unless something makes a later one urgent. Losses and the closing balance sheet carry forward, so each year builds on the one before (GOV.UK: carry forward Corporation Tax losses). Move a return up when HMRC has sent a determination, a loss claim nears its time limit, or its 18-month penalty date is close.

Taxley's decision table: which overdue return to deal with first

The order rarely changes the flat penalties, because each return's penalty depends on its own filing date and how late it arrives, not on what else you file (FA 1998 Sch 18 para 17). It changes the accuracy of the figures and which time limits you keep. A trading loss carried forward reduces later profits, so a later year filed first may need amending once the loss year is done; GOV.UK says amendments are usually made within 12 months of the filing deadline, which for an overdue year has often passed (GOV.UK: Company Tax Returns, making changes). Working forward from the oldest year avoids that. The exceptions in the table are deadlines that can't wait: a determination you want replaced with real figures, a claim with a statutory time limit, and the 18-month point at which unpaid tax starts to attract a percentage penalty. If none applies, oldest first is both the simplest and the most accurate order.

What penalties does each overdue CT600 carry?

Each return is penalised on its own. A return filed after its deadline gets a flat penalty of £200, or £400 once it's more than 3 months late, when its filing date is on or after 1 April 2026, and half those amounts when the filing date was earlier (Finance Act 2026 s.265). A tax-geared penalty can follow.

The flat penalty applies whether or not the company owes any tax, because the law ties it only to the failure to deliver the return by its filing date (FA 1998 Sch 18 para 17). The tax-geared penalty applies when a return still isn't filed 18 months after the end of its accounting period: 10% of the tax unpaid at that point, or 20% if the return arrives more than 2 years after the period ends (FA 1998 Sch 18 para 18). A year whose tax was paid in full by its 18-month point owes none. In GOV.UK's terms, that's £200 when the return is 1 day late, another £200 at 3 months, then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends) (GOV.UK: penalties for late filing). Each overdue year runs through that schedule from its own filing date, and what happens if you file your CT600 late covers it in full.

Does the three-late-returns rule change which you file first?

No. The flat penalty rises to £1,000, or £2,000 if more than 3 months late, for a third successive late return. That depends on whether the company was liable to a flat penalty for each of the two previous periods, not on the order in which you file them (FA 1998 Sch 18 para 17).

The rule needs three consecutive accounting periods within the charge to Corporation Tax, a return required for each, and a flat penalty for each of the first two. For returns whose filing date was before 1 April 2026, the third-in-a-row amounts were £500 and £1,000 (GOV.UK: increases to Corporation Tax late filing penalties). Once the run has started, every later return in it that's also late gets the higher amount, because each has two late returns before it. Filing the older returns now can't undo their lateness, so the higher penalty on a third late return is already fixed by the time you're catching up. What you can still control is the band: filing within 3 months of a return's filing date keeps it at the lower of the two amounts. The run ends when a return is filed on time, so the next return due after you've caught up is worth filing early (GOV.UK: penalties for late filing).

How does interest build up across several years of unpaid tax?

Separately for each year. Interest runs on each accounting period's unpaid Corporation Tax from the day after its own payment deadline, 9 months and 1 day after the period ends, until it's paid (GOV.UK: Corporation Tax interest charges). With similar amounts owed, the oldest unpaid year has built up the most.

HMRC's late payment rate for Corporation Tax is 7.75% a year from 9 January 2026, the Bank of England base rate plus 4% (HMRC interest rates). The interest is automatic, and it's deductible for Corporation Tax (GOV.UK: Corporation Tax interest charges). You don't have to file a return to pay its tax: use the 17-character payment reference for that accounting period, from HMRC's notice to deliver your tax return or the company's HMRC online account, because the reference changes with each period (GOV.UK: Pay Corporation Tax). Paying an old year's tax stops its interest. Paying a recent year's tax before its 18-month point also keeps that year out of the tax-geared penalty. If the company can't pay everything at once, ask HMRC about a payment plan, which HMRC agrees only if it's affordable (GOV.UK: if you cannot pay your tax bill on time). How to pay Corporation Tax covers the payment methods.

Why do losses make the order matter?

A loss changes the tax of the years after it. A trading loss that isn't used in its own year is carried forward and can be set against later profits while the trade continues, so a later year's return can't be right until the loss year's figures are settled (GOV.UK: carry forward Corporation Tax losses).

Two time limits can move a return up the list. A claim to set a carried-forward loss against a later period's total profits must be made within 2 years after the end of that later period, or within a longer period HMRC allows (CTA 2010 s.45A). A claim to carry a trading loss back against the profits of the previous 12 months must be made within 2 years after the end of the loss-making period, or a longer period HMRC allows (CTA 2010 s.37). When several years are overdue, either claim can be close to its limit, or past it, before the return that makes it is filed, so check those dates first. Losses made before 1 April 2017 follow older rules and can only reduce profits of the same trade. Trading losses brought forward: Corporation Tax and CT600 box 285 shows which box each loss goes in and how to work out the figure when no computation survives from the earlier years.

What if HMRC has already sent a tax determination?

File that year's return before its time limit. HMRC sends a tax determination when a return is 6 months late, saying how much Corporation Tax it thinks the company owes; you can't appeal against it, and you must pay the tax due and file the return (GOV.UK: penalties for late filing). HMRC then recalculates the interest and penalties.

A determination is HMRC's estimate, so it can be higher than the tax the company really owes. The company's own return replaces it, but only if the return is delivered within 3 years after HMRC's power to make the determination began, or within 12 months after the date of the determination, whichever is later (FA 1998 Sch 18 para 40). That power begins when the filing date passes without a return (FA 1998 Sch 18 para 36), so the 3 years run from roughly the filing deadline. A year with a determination and an old filing date is therefore the one to move up the list, even ahead of an older year without one. Paying the determined amount stops interest building on it, but it doesn't replace the return.

How much could three overdue returns cost?

In this example, £4,400 in penalties before interest. A company with a 30 June year end, whose returns up to the year to 30 June 2022 were on time, files the returns for the years to 30 June 2023, 2024 and 2025 on 30 November 2026, with £5,000 of Corporation Tax unpaid for each year.

Taxley's worked example: three overdue returns filed on 30 November 2026

The first two returns had filing dates before 1 April 2026, so their flat penalties use the old amounts: £200 each for being more than 3 months late. The third return's filing date, 30 June 2026, is after the change, and it's the third late return in a row, so its flat penalty is £2,000 (FA 1998 Sch 18 para 17). The 2023 and 2024 returns arrive more than 2 years after their periods ended, so each tax-geared penalty is 20% of the £5,000 unpaid at its 18-month point (FA 1998 Sch 18 para 18). The 2025 return arrives before 31 December 2026, its 18-month point, so it has none. Interest comes on top, from 2 April 2024, 2 April 2025 and 2 April 2026 respectively, the day after each payment deadline (GOV.UK: Corporation Tax interest charges).

The order matters most for the youngest year here. If its return slipped past 31 December 2026 with the tax unpaid, it would add £500 (10%), or £1,000 (20%) if filed after 30 June 2027. Paying its £5,000 by 31 December 2026 keeps it out of that penalty even if the return is late, because only tax unpaid at the 18-month point counts. The figures come from Taxley's free Corporation Tax late filing penalty calculator, which applies FA 1998 Sch 18 paragraphs 17 and 18 to one period at a time, so you can check each of your own years there.

Can Taxley file several years of overdue CT600s?

Yes, one return at a time, back to accounting periods that started on 1 April 2020. Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, charges one fee for each return.

Start with the oldest year. Once HMRC accepts a return, "Prepare next year's return" starts the next one with the company's details and last year's closing balance sheet as its opening position. The unused losses carry into it automatically, because the earlier return is filed and its period ends the day before; if the earlier return is still a draft, Taxley shows its carried-forward losses only as a suggestion for you to confirm. Profits before 1 April 2023 are taxed at the single rate that applied then, and a period straddling that date is split (GOV.UK: Corporation Tax rates). Taxley's capital allowances calculator starts at 1 April 2023, so you enter older years' allowances yourself. Taxley carries a trading loss back 12 months in the loss year's return, but not under the temporary three-year rule for periods ending between 1 April 2020 and 31 March 2022.

Each return is paid for separately, £44.50 (promotion price until 31 Dec 2026; £89.00 from 1 Jan 2027) with micro-entity accounts or £84.50 (promotion price until 31 Dec 2026; £169.00 from 1 Jan 2027) with small-company accounts, and Taxley files that year's accounts at Companies House once the return is paid and finished, unless Companies House already has them or they're first accounts that don't start on the incorporation date. Taxley doesn't file periods that started before 1 April 2020, and it doesn't pay HMRC for you. How to submit your Corporation Tax return online, step by step shows the process; take the 30-second check to see whether Taxley fits your company, or start your return.

Frequently asked questions

Do I have to wait for HMRC to accept one return before starting the next?

No, but it helps. Taxley automatically carries losses from the immediately preceding filed return, including one marked "filed elsewhere"; otherwise it suggests figures for confirmation. "Prepare next year's return" appears on a return once it's filed.

Do the late-filing penalties apply if the company owes no tax?

The flat ones do: £200 for a return due on or after 1 April 2026, or £400 if it's more than 3 months late (FA 1998 Sch 18 para 17). The tax-geared penalty is a percentage of unpaid tax, so it's nil when nothing is owed.

Which penalty amounts apply to a return that was due on 31 March 2026?

The old ones. The doubled penalties apply to returns whose filing date is on or after 1 April 2026 (Finance Act 2026 s.265), so a return due on 31 March 2026, for a year ending 31 March 2025, gets £100, or £200 if more than 3 months late.

Can I appeal the penalties on several late returns?

Yes, if the company had a reasonable excuse. GOV.UK says you must file the Company Tax Return before appealing its late filing penalty (GOV.UK: penalties for late filing), and you usually have 30 days from the date the penalty was issued (GOV.UK: appeal a penalty).

Can Taxley file a return for a period that started before 1 April 2020?

No. The earliest period Taxley files is one that started on 1 April 2020. An earlier period has to be filed another way, and HMRC's penalties and determinations apply to it in the same way (GOV.UK: penalties for late filing).

General information, not personalised tax or accounting advice.