Most of the opposition to May's proposed deal concerned the so-called Northern Irish "backstop", which attempts to avoid physical border checks between Northern Ireland, which is part of the UK, and the Republic of Ireland, which is part of the EU. Politicians want to avoid border infrastructure over fears it would reignite sectarian violence but efforts to sidestep a hard border call for a "temporary" arrangement that would see Northern Ireland remain aligned to some EU rules if another solution is not found by the end of the transition period in December 2020. Opponents say it could see the UK trapped in EU purgatory with the UK bound by EU rules but unable to influence them, and that it could continue indefinitely.

Former Brexit secretary Dominic Raab, an outspoken critic of May, told the BBC hardline Brexiteers would support May's deal if the backstop issue was dealt with.

May started Tuesday by addressing her Cabinet and urging senior ministers to get behind her deal. Environment Secretary Michael Gove told Radio 4's Today program the deal was preferable to leaving with no deal, "or worse, no Brexit at all".

"If we don't vote for this agreement, then we risk playing into the hands of those who do not want Brexit to go ahead," he said.