Managing Director, HC-Customs Limited · Customs compliance specialist · Brexit formalities · Updated January 2026
4 years
HMRC audit window under CEMA 1979
87%
Of audits find at least one irregularity
£100K+
Average C18 demand in a compliance failure
30 days
Typical response deadline once notified
What is an HMRC customs audit?
An HMRC customs audit — formally a post-clearance examination — is an inspection of your import or export declarations after goods have already been released from customs control. Unlike a pre-clearance intervention (where goods are held at the border), a post-clearance audit happens weeks, months, or even years after shipment.
HMRC carries out these checks under powers in the Customs and Excise Management Act 1979 (CEMA) and the Taxation (Cross-border Trade) Act 2018. They can request any records, correspondence, and supporting documents related to your import activity — and they can go back up to 4 years from the date of the declaration.
If HMRC discovers an error — an under-declared customs value, a wrong commodity code, or a missing proof of origin for a preferential tariff — they can issue a C18 Post Clearance Demand Note requiring you to pay the additional duty, plus interest. In serious cases, penalties apply on top.
What triggers an HMRC customs audit?
HMRC uses risk-based targeting to select businesses for audit. Common triggers include:
Duty anomalies
Declarations where the declared customs value appears low relative to typical market prices for the commodity code.
High-value preference claims
Claiming preferential tariff rates (0% under a UK FTA) repeatedly without adequate proof of origin on file.
IP/OP discharge failures
Failure to discharge Inward Processing or Outward Processing authorisations within the required period.
Industry sector signals
HMRC runs sector-wide campaigns — textiles, electronics, food, and automotive parts have all been targeted in recent years.
Agent or freight forwarder flags
If your customs agent is under investigation, all businesses they filed for may be reviewed.
Random selection
Some audits are random, regardless of compliance history, as a deterrent and control measure.
Whistleblower reports
Competitor or employee disclosures that prompt HMRC to investigate a specific importer.
What does HMRC check during an audit?
HMRC auditors will typically request some or all of the following for each declaration under review:
Customs value evidence
Customs duty is calculated on the CIF (Cost + Insurance + Freight) value of imported goods. HMRC will compare your declared customs value against your commercial invoices, purchase orders, supplier contracts, and bank payment records. Any discrepancy — including discounts, royalties, or proceeds-of-resale arrangements not included in the declared value — can result in a C18 demand.
Commodity code accuracy
Each commodity code determines the duty rate applied. HMRC checks whether the code declared matches the actual goods, using product specifications, technical datasheets, and physical inspection if necessary. Incorrect classification — even if unintentional — generates a liability for the difference in duty.
Proof of preferential origin
If you claimed a preferential tariff rate under a UK FTA (such as the UK-EU TCA or UK-Japan CEPA), HMRC will ask to see the proof of origin document — a Statement on Origin, EUR.1 movement certificate, or REX declaration. If you cannot produce it, the preference is denied and duty is calculated at the MFN rate instead.
Import licences and controls
HMRC checks that all required licences (OGELs, SIELs, phytosanitary certificates, CITES permits) were valid at the time of importation and that their conditions were met.
What is a C18 Post Clearance Demand Note?
A C18 is HMRC's formal demand for duty that should have been paid at the time of importation but was not — due to an error, omission, or fraud. It can be issued up to 4 years after the original declaration (3 years in cases of negligence, 10 years in cases of fraud).
A C18 typically includes the underpaid duty plus customs interest (currently charged at the Bank of England base rate + 2.5%). If HMRC determines the error was due to negligence or deliberate behaviour, civil penalties under Finance Act 2003 Schedule 24 apply on top — up to 100% of the unpaid duty.
You have 30 days to pay or appeal. An appeal is made to the HMRC Review team first, and then to the First-tier Tribunal (Tax Chamber) if unresolved. A strong contemporaneous evidence pack — created at the time of importation, not retrospectively — is your best defence.
How to build an HMRC audit defence pack
The most effective audit defence strategy is preparation before HMRC contacts you. For every import declaration, you should retain:
Records must be retained for a minimum of 4 years from the date of import (HMRC Notice 143). Businesses under an IP/OP authorisation must retain records for the duration of the authorisation plus 4 years.
HC Customs Platform
Build your audit defence pack automatically — for every declaration
HC Customs connects directly to HMRC CDS via OAuth2. For every MRN it pulls live, the platform automatically builds a structured evidence record — customs value check, commodity code match, document completeness, and preference origin status — archived and ready for HMRC inspection.
Frequently asked questions
How long does HMRC have to audit a customs declaration?
HMRC has 3 years from the date of acceptance of the declaration in cases of negligence (or where you self-correct via voluntary disclosure), and up to 4 years in other cases. In cases of fraud, the window extends to 10 years.
Can I appeal a C18 Post Clearance Demand Note?
Yes. You can request a Statutory Review within 30 days of the C18 being issued. If the review upholds the demand, you can appeal to the First-tier Tribunal (Tax Chamber). You should engage a customs duty specialist for any dispute above £10,000.
What is voluntary disclosure and does it reduce penalties?
A voluntary disclosure is where you identify an error in your own declarations and notify HMRC before they contact you. Voluntary disclosure typically results in reduced or zero penalties (though the duty itself is still owed). HC Customs can help identify declaration anomalies before HMRC does.
Do I need a customs audit if I use a freight forwarder?
Yes. Even if your customs agent made the declaration, you (as the importer of record) are legally responsible for its accuracy. The agent acts on your instructions — which is why maintaining a clear Customs Clearance Instruction for every shipment is essential.