Understanding the TSS to CDS Data Flow: A Technical Guide

If you work anywhere near UK customs data, you will hit TSS and CDS. Here is what actually happens on the wire.

System roles

CDS (Customs Declaration Service) is HMRC's declaration processing platform. It handles imports, exports, pre-lodgement, amendments, document uploads and duty deferment accounts. It replaced CHIEF on 31 March 2023.

TSS (Trader Support Service) is a free web portal that acts as a validation and submission layer specifically for Northern Ireland movements.

The flow

1. The trader enters movement details in the TSS portal.
2. TSS validates fields against CDS-compatible rules (commodity code format, EORI validity, procedure codes, valuation).
3. TSS formats the declaration into the structure CDS expects.
4. TSS submits the declaration into CDS on the trader's behalf.
5. HMRC processes the declaration inside CDS.
6. The outcome — clearance, query or amendment — returns to the trader. For a fuller architectural view of the two systems, this TSS and CDS workflow guide walks through how the upstream/downstream split affects data ownership.

Identity and access

CDS requires a GB or XI EORI, Government Gateway credentials, Unique Taxpayer Reference and business address details. Sole traders also need a National Insurance number. TSS registration only asks for an EORI and business details.

Practical implications

If you build integrations, target the shape of the data TSS expects — because that is what CDS ends up receiving. If you scale beyond Northern Ireland, plan for a direct CDS connection through certified customs software.

Explore the technical view of a live CDS integration. Watch a demo with iCustoms.