HMRC penalty letters explained: what every dormant-company letter actually means

By DormantFile · Updated 27 May 2026

If you have just opened an HMRC letter and your stomach is in your mouth — this guide is the reference. It catalogues every common piece of HMRC correspondence a dormant UK company can receive, what it actually means, what HMRC wants you to do, and how serious it is.

For the broader "I got a letter — calm me down" piece, see HMRC letter about your dormant company: what to do. This guide assumes you know you have a letter and you want to know what kind.

How to identify which letter you have

HMRC letters carry a reference code in the top-right corner. Common codes for dormant-company correspondence include:

• CT603 — Notice to deliver a Company Tax Return
• CT41G — New company information form (issued shortly after incorporation)
• SA316 — Self assessment notice (usually a misdirected letter; not relevant for limited companies)

Late filing penalty notices are the exception — they are headed "Notice of penalty determination" rather than carrying a memorable form code. If your letter cites an accounting period and a penalty amount, that is what you have.

If you cannot find a reference code, look at the heading or subject line. HMRC letters almost always state their purpose in the first paragraph.

CT603 — Notice to deliver a Company Tax Return

What it is: HMRC's formal demand that you file a CT600 for a specific accounting period.

What it looks like: A letter or email saying "Notice to deliver a Company Tax Return" followed by an accounting period (e.g. "1 April 2025 to 31 March 2026").

What it means: HMRC is requiring a CT600 covering that period. The obligation is legal — you must file or be in breach of statutory duties.

How serious: Standard. The CT603 is HMRC's normal annual reminder, not a penalty. But ignoring it triggers a late filing penalty after the filing deadline.

Deadline: 12 months after the end of the accounting period (the filing deadline). If you have tax to pay, the payment deadline is 9 months and 1 day after the end of the period, which is different.

How to respond:

1. Confirm whether the company is genuinely dormant for the period
2. If dormant, file a nil CT600 before the deadline
3. If not dormant, file a regular CT600 (likely with an accountant)
4. Once HMRC has accepted the return, the CT603 obligation is satisfied

Receiving multiple CT603s in successive years is normal until you have HMRC's confirmation of dormant CT status.

Late filing penalty notice — late CT600

What it is: A penalty for filing your CT600 late, or not filing it at all by the deadline.

What it looks like: A letter headed "Notice of penalty determination" or similar, citing your accounting period and a penalty amount.

What it means: You missed the filing deadline on the CT603, and HMRC has now imposed an automatic penalty.

How serious: Moderate. The penalties accrue:

• £100 — automatically issued the day after the deadline passes
• A further £100 — if the return is more than three months late
• 10% of the unpaid tax — at six months late, HMRC estimates your Corporation Tax bill and adds 10% of the unpaid amount
• A further 10% — if the return is still unfiled at twelve months

If you file late three years in a row, the two £100 penalties rise to £500 each. There are no daily penalties for a late CT600 — the £10-a-day regime you may have read about belongs to Self Assessment, not Corporation Tax.

For a nil return, the tax-geared penalties are 10% of zero — nothing. So a dormant company's real exposure is the fixed £200 per period (£100 + £100), or £1,000 per period if it is on its third consecutive late return.

Deadline: None specific to the penalty notice itself, but the underlying CT600 is now overdue and the penalties escalate until you file.

How to respond:

1. File the missing CT600 immediately — this stops further penalties accruing
2. Pay the issued penalty within 30 days, or you may incur interest
3. If you believe the penalty was issued in error (e.g. HMRC's records show the wrong dormancy status), respond in writing within 30 days with a formal appeal
4. If the company is fully nil and the missed filings span multiple years, file everything outstanding and consider writing to HMRC to request the penalties be mitigated — they sometimes will, particularly if you bring everything up to date promptly

CT41G — New company information

What it is: A form HMRC sends to every newly incorporated company asking for basic Corporation Tax registration information.

What it looks like: A multi-page form with sections for trading start date, accounting period dates, registered office, and director details.

What it means: HMRC is setting up your Corporation Tax record. They need to know when (if at all) you started trading.

How serious: Standard. Ignoring it can mean HMRC defaults your trading start date and your accounting period — potentially creating a mismatch with what you intend to file.

Deadline: Within 3 months of the company becoming active (starting to trade or otherwise coming within the charge to Corporation Tax) — not 3 months from incorporation. HMRC is often flexible if you respond before the first accounting period ends.

How to respond:

1. If the company has started trading, complete the form with your actual trading start date
2. If the company is dormant from incorporation and you do not plan to trade, write to HMRC stating that the company is dormant and not yet trading — this can prevent CT603 notices being issued
3. Return the form to the address shown

Penalty mitigation — when HMRC will reduce penalties

HMRC will sometimes mitigate or remove penalties, particularly for first-time offenders, fully-nil situations, or where there is a "reasonable excuse". Acceptable reasons include:

• Serious illness affecting the director
• Bereavement
• Unexpected hospital stay
• HMRC service failure (e.g. you tried to file but the HMRC API was down)
• Software failure (where you can document attempting to file on time)

What does not count as a reasonable excuse:

• You forgot
• You did not know you had to file
• You did not receive the CT603 (HMRC takes the position that posting it is sufficient delivery)
• Your accountant failed to file on your behalf
• You could not afford to pay

If you have a genuine reasonable excuse, appeal in writing within 30 days of the penalty notice, citing your reasons and any supporting evidence.

What to do if you have multiple unfiled years

If you have ignored CT603 notices for multiple years and are now staring at a stack of penalty notices, the priorities are:

1. Stop the bleeding. File the oldest overdue CT600 first, then work forwards to the present. Every period you file stops the penalty clock escalating on that period.
2. Don't make the situation worse. Do not write to HMRC explaining your situation before filing. Get the returns in first, then address the penalties.
3. Bring everything current. File every outstanding period, including the current open one.
4. Then write to HMRC. Once everything is up to date, write a single letter outlining the circumstances, requesting penalty mitigation, and confirming the company is now fully compliant.
5. Set up reminders for the future. Subscribe to a calendar feed of deadlines so you do not repeat the situation.

Filing software like DormantFile can handle outstanding CT600s for any historical period the company existed for. There is no separate "catch-up" mechanism — you just file each missed period as a nil return.

Letters you might receive that are NOT about your dormant company

Some letters look like they are about your dormant company but are actually about something else:

Self assessment letters (SA316, SA300, etc.) — These are for personal tax returns, not limited companies. If you receive one in the company's name, it is usually a misdirected letter. Check the addressee carefully. (One related point: if you file a personal return yourself, from 2025/26 your dormant directorship must be declared on it — see the new SA102 rules.)

VAT correspondence — If your company is not VAT-registered, any VAT letter is either a mistake or someone using your company name on a VAT return. Check Companies House for company names similar to yours.

PAYE correspondence — If your company is not registered as an employer, PAYE letters should not be addressed to it. Same advice: verify the addressee and the company name.

Companies House letters — These come from Companies House, not HMRC. Common ones include strike-off warnings, confirmation statement reminders, and accounts overdue notices. They are separate from any Corporation Tax matter.

When to use a professional

Most dormant-company HMRC correspondence is straightforward and can be handled directly. Get professional help (accountant or tax adviser) if:

• The penalties run to several thousand pounds across multiple years
• HMRC has opened a formal enquiry into the company (an enquiry opens with a written enquiry notice under Schedule 18 of the Finance Act 1998, typically followed by information requests)
• The company has some trading history that you are not sure about — there may be tax due
• You are reactivating the company and need a clean transition
• HMRC has issued a determination of tax owed (a "best judgment" assessment) and you need to challenge it

For everything else — including all standard nil CT600 filings and routine penalty appeals — the DIY route is fine and far cheaper.

Key points

• Most HMRC letters about Corporation Tax carry reference codes (CT603, CT41G). The code tells you exactly what the letter is; penalty notices state their purpose in the heading instead.
• A CT603 is HMRC asking you to file a CT600. Standard, annual, no penalty yet.
• A late filing penalty notice means you missed the CT600 deadline. For a nil return the exposure is £100, rising to £200 past three months — £500/£1,000 for a third consecutive late return. There are no daily penalties.
• A CT41G is HMRC setting up your Corporation Tax record. Respond promptly so HMRC has accurate dates.
• Penalties can be mitigated for genuine reasonable excuses, but "I forgot" or "I didn't know" do not count.
• If you have multiple unfiled years, file everything outstanding first, then write to HMRC about mitigation.
• Letters about VAT, PAYE, Self Assessment, or Companies House matters are not Corporation Tax letters and need separate handling.

If you need to file a nil CT600 or a stack of missed ones, DormantFile generates compliant returns for any historical period and submits them directly to HMRC. See how it works for the full flow.

For more on CT600s, dormant tax obligations, and HMRC correspondence, the CT600 hub collects every relevant guide, answer, and tool on this site.