Firms in Northern Ireland will have to submit declaration forms for goods heading to the rest of the UK, under the government’s Brexit deal.
Brexit Secretary Steve Barclay confirmed the details on Monday when giving evidence to the Lords Exiting the EU committee.
This followed previous assurances that NI-GB trade would be “unfettered”.
DUP MP Sammy Wilson said it represented a “clear breach” of previous commitments made by the government.
Mr Barclay was correcting a previous answer he had given to the committee, having initially said he believed the forms would not be required.
However, it confirms extra paperwork would be required for trade between Northern Ireland and Great Britain, as part of Boris Johnson’s new Brexit deal.
The Democratic Unionist Party (DUP) has already said it would not support the plan because it leaves NI subject to different rules than GB.
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European Commission rules state exit summary declarations are needed when goods leave the EU’s customs territory, but regular customs declarations are not required.
In December 2017, the joint report agreed by the UK and EU as they began Brexit negotiations stated: “In all circumstances, the United Kingdom will continue to ensure the same unfettered access for Northern Ireland’s businesses to the whole of the United Kingdom internal market.”
Labour peer Stewart Wood, who had asked Mr Barclay the question, said it was clear there would be a “very real commercial border” between Northern Ireland and Great Britain.
On Tuesday, the government is beginning the final process of trying to push its Brexit deal through Parliament.
MPs will be asked to vote on the legislation to implement Brexit – known as the Withdrawal Agreement Bill (WAB).
The 110-page document was published on Monday night and the government wants to get the entire bill through Parliament in three days.
If they back his deal, they will then be asked to approve an intensive three-day timetable in which to consider the legislation.
MPs have criticised the government for its proposed timetable, saying it is not long enough to scrutinise the details of the bill.
On Saturday, the DUP helped inflict defeat on the government by voting to delay Brexit until the WAB has been passed in Parliament.
Mr Wilson had said the party would also vote against the WAB in principle on Tuesday.
If the government loses Tuesday’s votes, it will be harder for it to meet its Brexit deadline of 31 October.
Downing Street has been contacted for comment.