EU chiefs have left out crucial compromise language, as secured by Theresa May, from the draft legal text of Britain’s withdrawal from the EU, which will be published on Wednesday.
The exit treaty omits wording by Britain promising that “no new regulatory barriers” will be developed between UK mainland and Northern Ireland.
A UK government official told the Financial Times: “It is no surprise that the EU will continue to produce documents that push their negotiating position.
“However it is important they accurately reflect the December divorce deal and include the full scope of the agreement, rather than simply the bits which best suit one side.”
The treaty will outline a last-resort option for Northern Ireland to remain under the EU’s regulatory regime so a hard border can be avoided.
It comes after Mrs May and her British negotiators conceded ground on the Irish border in a bid to kickstart trade talks with the EU.
But critics at the time feared the agreement was a fudge as there had been little real movement on the Irish border issue and the arguments that nearly torpedoed the deal would simply rumble on into Phase 2.
The agreement promised there would be no hard border and to uphold the Belfast agreement and made clear the whole of the UK, including Northern Ireland, would be leaving the customs union.
It gave no details of how an open border will actually work but said in the absence of a later agreement, the UK will ensure “full alignment” with the rules of the customs union and single market that uphold the Good Friday agreement.
The UK agreed that continuing “alignment” on all single market and customs union regulations that, if removed, could lead to a hard border.
An agreement between the UK and EU collapsed at the 11th hour back in December when the DUP objected to plans for “regulatory alignment” between Northern Ireland and the Republic to maintain a soft border between the two.
But DUP leader Arlene Foster said six “substantial changes” had been made to the Brexit deal regarding the Irish border before Mrs May and Jean-Claude Juncker unveiled the breakthrough agreement.