Theresa May is facing near-certain defeat in her final Brexit gamble, as both Tory rebels and the Democratic Unionist Party vowed to vote down crucial legislation.

Brexiteer Conservatives and the Northern Ireland Unionists combined to insist the withdrawal agreement bill – which the prime minister will finally bring forward in June – remained “unacceptable”.

With Labour also saying it will not support the bill without significant further concessions, it is facing a heavy defeat which is likely to accelerate Ms May’s departure from No 10.

Nigel Dodds, the DUP's Westminster leader, reacted to the potentially-dramatic shift in strategy by asking simply: “What has changed?”

“Unless she can demonstrate something new that addresses the problem of the backstop then it is highly likely her deal will go down to defeat once again,” he said.

Making clear the DUP still opposed the Irish backstop element of the agreement, he added: “For the bill to have any prospect of success, then there must be real change to protect the economic and constitutional integrity of the United Kingdom.”

Owen Paterson, a leading Brexiteer, said: “Sadly, we will vote against it again. It doesn’t change the essential nature of the withdrawal agreement, which is unacceptable.”

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The former cabinet minister pointed to the rise of Nigel Farage’s Brexit Party as evidence of the need to stick to a willingness to “leave with no deal”.

“People aren’t flocking to this new party’s rallies from the Tory party and the Labour party, in their tens of thousands, clamouring for the agreement.

“They are going because they want to leave, they voted to leave and they thought we would leave on 29 March – and they feel thwarted and cheated by the main parties.”

Many at Westminster are perplexed by the decision to plough ahead with the bill, after No 10 repeatedly signalled it would not do so without an agreement with Labour – something now less likely than ever.

It appears to be a strategy to at least buy the prime minister a little more time when she meets the executive of the backbench 1922 Committee on Thursday.

It has demanded a timetable for her resignation, threatening to change party rules to allow a summer vote of no confidence if she refused, but can now be expected to wait until next month.

Downing Street is also braced for a further backlash when the bill is actually published, in the week beginning 3 June, and its controversies are set out in full detail.

It will hope to woo Labour MPs by offering a customs arrangement with the EU, but only until the next election – falling short of Labour’s demands for a permanent customs union.