Guides / T1 guarantees and office codes · Updated 19 October 2026

T1 transit guarantees and NCTS office codes: how they're worked out

Our general transit document (T1 & T2) guide and our transit guarantees overview already explain individual versus comprehensive guarantees and the NCTS lifecycle. This guide goes narrower: how the guarantee amount behind a single T1 movement is actually calculated, and what the office-of-departure and office-of-destination codes buried inside the movement's MRN really stand for.

If you have not yet decided whether your load needs a T1 at all, start with our guide on T1 transit from the UK to the EU; for the individual-versus-comprehensive guarantee choice and the guarantee reference number and access code, see transit guarantees and NCTS explained; and for T1 versus T2 status itself, see T1 vs T2. None of those three cover the calculation method or the MRN's office codes in detail, which is what follows.

How the guarantee amount for a movement is actually calculated

A transit guarantee, whether it is an individual guarantee for one movement or a slice of a Customs Comprehensive Guarantee, has to cover the "potential debt" a movement could create if the goods never arrive. GOV.UK's guidance on calculating that amount sets out a specific method, not a flat percentage of the goods' value:

1. Classify the goods first. You need the correct commodity code for each type of goods in the movement, because the code is what determines the rate used in the next step.
2. Use the highest rate, not the rate you expect to pay. GOV.UK is specific that you must use "the highest rate of UK Customs Duty, VAT, excise duty" for each type of goods, applied as third country import rates, meaning the rate that would apply if the goods had no preferential origin at all. This is deliberately conservative: the guarantee has to cover the worst case, not the duty you actually expect to owe at destination.
3. Only use a lower, preferential rate in narrow cases. GOV.UK says it is "not appropriate to use alternative import duty rates" just because your declared route does not plan to pass through other territories. If the movement could in practice be diverted to another Common Transit Convention country, you must guarantee at UK third country rates. The exception is where goods genuinely cannot leave a defined area, such as certain Great Britain–Northern Ireland or Northern Ireland–EU movements, where the highest rate on arrival in that specific destination may be used instead.
4. Convert to euros at the official rate. Where a value is not already expressed in euros or the currency of the country of departure, GOV.UK requires "the official exchange rate when converting pound sterling and other national currencies into or from the euro."
5. Fall back on a fixed figure only if you genuinely cannot calculate the debt. If an accurate figure cannot be determined, GOV.UK allows the guarantee to assume "the sterling equivalent of 10,000 euros" for that movement. This is a fallback for genuine uncertainty, not a default to reach for out of convenience; if it does not reflect a reasonable estimate, GOV.UK expects you to agree a different calculation with HMRC instead.

For a comprehensive guarantee covering many movements at once, each new movement's calculated amount is held against the overall reference amount until that movement is discharged at destination, at which point it is credited back. That mechanic, and the difference between an individual guarantee and a Customs Comprehensive Guarantee, is covered in full in our separate transit guarantees and NCTS guide.

What an MRN's office codes actually represent

Every transit movement carries a movement reference number once it is accepted into NCTS, and that number is not an arbitrary string. GOV.UK's Transit Manual Supplement breaks down its structure using a worked example, the MRN 22GB00005112345678:

• The first two characters are the year the movement was registered. In the example, 22.
• The next two characters are the country code of the office of departure. In the example, GB.
• The next six characters are the code for the office of departure itself. GOV.UK's example states that 000051 is the code for Felixstowe.
• The remaining characters are a unique reference for that specific movement.

So an office code is not a postcode or a free-text name; it is a fixed numeric reference tied to one specific customs office, and it is embedded directly in the MRN rather than sitting in a separate field you have to look up each time. Once you know the pattern, you can read the office of departure's country and identity straight off the MRN without opening the declaration.

Northern Ireland movements are the one place this pattern changes at the country-code position. GOV.UK states that for movements starting in Northern Ireland, the office of departure code begins with XI, not GB, even though Northern Ireland is part of the UK's customs territory for other purposes. If you see an MRN starting XI rather than GB, that is telling you the movement started in Northern Ireland, not that something has been mis-keyed.

The office of destination does not appear inside the MRN itself; it is simply the office named on the declaration that the goods must be presented to, and which records the arrival that closes the movement. GOV.UK points traders to the European Commission's customs office database to search for a specific office by country, region, city or reference number. That database does not make explicit whether its "reference number" matches the departure-code format embedded in an MRN, so confirm an office's exact reference with your office of departure or software provider rather than assume the two are identical.

Putting the two together

These two pieces of information answer different questions. The guarantee calculation tells you, before the movement starts, how much guarantee capacity a given load will tie up. The office code embedded in the resulting MRN tells you, once the movement exists, exactly which office accepted it and when, useful when chasing an overdue arrival or querying an uncredited guarantee balance.

• Classify goods correctly and use the highest third-country rate before assuming a movement's guarantee cost
• Only use a preferential rate where goods genuinely cannot leave a defined UK/Ireland or UK/EU area
• Treat the 10,000 euro assumption as a fallback for genuine uncertainty, not a shortcut
• Read the year and office-of-departure country and code straight from the first ten characters of the MRN
• Remember Northern Ireland movements show XI, not GB, at the country-code position

Easy Clearance, a UK–EU customs clearance agency handling import and export declarations, T1/T2 transit and UK–Poland removals, has published its own guide to T1 guarantees and NCTS office codes, run by the same team as Easy Clearance.

Related guides

• Transit guarantees and NCTS explained

• T1 vs T2: the difference between transit documents

• T1 transit from the UK to the EU: what the broker does

Sources

• Check the amount of guarantee needed for common and Union transit (GOV.UK)
• Transit Manual Supplement, section 4: Common/Union transit (GOV.UK)
• Moving goods out of Great Britain using transit: step by step (GOV.UK)

General information, not legal, tax or customs advice. Rules change; confirm current requirements on gov.uk or with a registered agent before you ship.