If you discover that an import declaration understated the duty or import VAT due, you tell HMRC using form C2001 — an application for a voluntary clearance amendment (underpayment). HMRC then raises a C18 charge for the additional amount owed.
You can make a voluntary disclosure at any time. And here is the part that decides whether this is a manageable problem or an expensive one:
An unprompted disclosure of a careless inaccuracy can have its penalty reduced to nil.
That is not a sales pitch. It is in HMRC’s own Compliance Handbook. Which means the difference between finding an error and disclosing it, versus finding an error and sitting on it, can be the entire penalty.
Unprompted vs prompted: the distinction that matters
HMRC draws a hard line here, and everything else follows from it.
A disclosure is unprompted if you make it at a time when you have no reason to believe that HMRC has discovered, or is about to discover, the inaccuracy.
If you have reason to think HMRC is onto it — a compliance check letter has arrived, questions have started, an assessment is in the post — the disclosure is prompted, and the available penalty reduction is significantly smaller.
The practical translation is blunt:
| When you disclose | Status | Penalty position |
|---|---|---|
| Before HMRC has any idea | Unprompted | Maximum reduction — can reach nil for a careless error |
| After a check has opened or questions started | Prompted | Reduction available, but materially less |
The window for the best outcome closes the moment HMRC starts looking. If you have found something, the clock is already running.
What “careless” means, and why it matters
Penalties scale with behaviour, not just with the amount:
- A reasonable mistake, where you took reasonable care — potentially no penalty at all
- Careless — you did not take reasonable care, but there was no intent to mislead
- Deliberate — you knew, and the treatment is far more serious
Most customs errors are careless rather than deliberate: the wrong commodity code applied consistently, a preference claimed without checking the origin evidence, freight omitted from the customs value. Genuinely careless errors, disclosed unprompted, are where the reduction to nil becomes realistic.
Deliberate evasion is a different matter entirely and this article is not the right guide for it. If there is any possibility that what you have found was deliberate rather than careless, take professional advice before you contact HMRC at all. How that is handled from the first communication onwards materially affects where it ends up.
How to disclose
1. Establish the full extent of it first. This is the step people skip, and skipping it is costly. If a commodity code was wrong on one declaration, it was probably wrong on every declaration of that product. Disclose the whole pattern, not the one entry you happened to notice — a partial disclosure that HMRC then extends for you looks very different from a complete one you brought yourself.
2. Quantify it. Work out the additional duty and import VAT for every affected entry. You will need the MRNs.
3. Complete form C2001. State the reasons for the underpayment and supply supporting evidence, including any relevant technical documentation — a product specification establishing the correct classification, for instance.
4. Explain how it happened and what you have changed. Not strictly required, but HMRC assesses the quality of disclosure, and a clear account of cause and remedy is part of that. “We have since implemented a classification review” is worth more than silence.
5. HMRC raises a C18. This is the formal charge for the additional duty and VAT. Paying it promptly is part of the co-operation HMRC weighs.
6. Co-operate with follow-up questions. Full co-operation is explicitly one of the conditions for maximum penalty reduction.
Note there is a separate route for declarations originally made in CHIEF rather than CDS.
What you get for disclosing well
HMRC gives maximum reduction for quality of disclosure where you:
- Notify HMRC of your intention to disclose
- Make a full disclosure, including paying what is owed by the deadline
- Co-operate fully with requests for further information
Those three are the whole test. Notice that none of them is about how large the error was — the reduction is earned by behaviour after discovery, which is entirely within your control.
The argument for disclosing, put plainly
Businesses hesitate for understandable reasons: it feels like inviting scrutiny, and there is always a hope the error goes unnoticed.
Three things make that hope a bad bet:
HMRC has the same data you do. Your declarations are in their systems. Risk profiling looks for exactly the patterns a systematic error creates — inconsistent codes, preference claims from unlikely origins, values outside the norm for a commodity code.
The window is four years. HMRC can assess underpaid duty across a four-year period. An error made two years ago is still well inside it, and every month it continues, the eventual assessment grows.
Disclosure is the only route to the best outcome. Once a compliance check opens, the unprompted option is gone permanently. There is no way back to it.
The realistic comparison is not “disclose and pay” against “say nothing and pay nothing”. It is “disclose now, potentially with no penalty” against “be found later, with a larger assessment and a penalty that is harder to reduce”.
While you are looking, check the other direction
A review that surfaces underpayments very often surfaces overpayments too — the same sloppy classification that underpaid on one product may have overpaid on another.
Those can be reclaimed on a C285 within three years. It is not unusual for a properly conducted review to produce a net position considerably better than the business feared when it started.
Which is an argument for doing the review deliberately rather than discovering things one panicked entry at a time.
Finding errors before HMRC does
Everything above depends on you finding the error first. That is the whole game — unprompted versus prompted is decided by who noticed.
You can do this yourself: request your declaration data from HMRC’s Get Customs Data service, covering four years free of charge, and reconcile it against your purchase records looking for classification inconsistency, preference claims you cannot evidence, and values that do not match your invoices.
The HC Customs compliance portal does that continuously instead of periodically. It pulls your CDS data as an authorised third party, reconciles every declaration against your own purchase and goods-received records, and flags anomalies as they occur — so errors surface in weeks rather than being discovered when a compliance check letter lands, at which point the unprompted window has already closed.
30-day free trial, no card required.
Frequently asked questions
How do I tell HMRC about a customs underpayment?
Use form C2001, an application for a voluntary clearance amendment (underpayment), for declarations made in the Customs Declaration Service. HMRC then issues a C18 charge for the additional duty and VAT. A separate route applies to declarations originally made in CHIEF.
Will I be penalised for disclosing a customs error?
Not necessarily. An unprompted disclosure of a careless inaccuracy can have the penalty reduced to nil, provided you notify HMRC, make a full disclosure including payment, and co-operate fully. A prompted disclosure — after HMRC has started looking — attracts a smaller reduction.
What counts as an unprompted disclosure?
One made when you have no reason to believe HMRC has discovered, or is about to discover, the error. Once a compliance check has opened, disclosure is prompted.
How far back do I need to go?
HMRC can assess underpaid duty across four years, so a full disclosure should cover the whole period the error affected, within that window.
Should I disclose a very small underpayment?
Small individual errors often indicate a systematic cause, and the systematic version is rarely small. Establish the full extent before deciding — the question is usually not “is this one entry worth disclosing” but “how many entries does this affect”.
Do I need professional advice?
For a straightforward careless error with a clear cause, many businesses disclose without help. If the amount is large, spans many entries, or there is any question of it being deliberate rather than careless, take advice before contacting HMRC — early handling materially affects the outcome.