A group of Brexit-supporting MPs asked by their colleagues to examine changes to Britain's EU divorce deal recommended that parliament reject it in a vote on Tuesday.
"In the light of our own legal analysis and others we do not recommend accepting the government's motion today," Conservative MP Bill Cash said on behalf of the group.
They include one lawyer, six Conservative MPs plus Nigel Dodds, deputy leader of the Northern Ireland's Democratic Unionist Party (DUP).
Without the support of the DUP's 10 MPs, the Brexit deal is unlikely to pass when the House of Commons votes on it on Tuesday evening.
Prime Minister Theresa May and the EU have agreed fresh guarantees on the operation of the Irish border "backstop" plan in the deal, which risks keeping Britain indefinitely in a customs union with the bloc.
Attorney General Geoffrey Cox told MPs that her changes reduce the risk that Britain will be trapped in the backstop in the event that the EU refuses to exercise "good faith" in the talks to find an alternative to the backstop.
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But he admitted it could still be trapped if the talks to find an alternative simply break down.
Dodds told MPs: "From what the attorney general is saying today, provided there is no bad faith, the fact is that Northern Ireland and the rest of the UK could be trapped.