Theresa May has postponed the final vote on her Brexit deal after a last-minute conference call with cabinet ministers, a clear admission by the prime minister that she does not believe she can get the unpopular EU withdrawal agreement through the Commons.

The drastic move, as May faced defeat by up to 100 of her own MPs, is a humiliation for the prime minister, whose aides and cabinet ministers had emphatically denied there was any prospect of pulling the vote, even as late as Monday morning.

Advisers, ministers and loyalist MPs had been urging May to delay Tuesday’s vote in order to seek new assurances from Brussels and avoid the chaos of a parliamentary defeat.

May is expected to confirm the vote has been delayed in an oral statement to the House of Commons at 3.30pm.

It will be immediately followed by a business statement from the leader of the House of Commons, Andrea Leadsom, implying that she will confirm the procedural details of the postponement.

The vote could take place next week or even be delayed until early January, although this would allow less time for the ensuing Brexit legislation to be passed through parliament before 29 March. The ultimate deadline for the vote is 21 January.

News of the delay caused the pound to tumble to its lowest level in 18 months.

The prime minister spoke to cabinet colleagues on Monday morning in a hastily arranged conference call, after apparently concluding she could not win over enough of the 100 Tory MPs who had said they were against the deal.

Before that call, a cabinet source said they believed ministers would back the decision to delay the vote, but added: “She needs to have a bloody good plan B.”

The children’s minister, Nadhim Zahawi, said on Twitter that May had “listened to colleagues” and would now “head to Brussels to push back on the backstop”.

Olly Robbins, the prime minister’s Brexit negotiator, arrived in Brussels on Monday. EU sources said Downing Street was seeking extra safeguards that the “backstop” solution in the withdrawal agreement would never be triggered.

Quick Guide Brexit and backstops: an explainer Show A backstop is required to ensure there is no hard border in Ireland if a comprehensive free trade deal cannot be signed before the end of 2020. Theresa May has proposed to the EU that the whole of the UK would remain in the customs union after Brexit, but Brussels has said it needs more time to evaluate the proposal. As a result, the EU insists on having its own backstop - the backstop to the backstop - which would mean Northern Ireland would remain in the single market and customs union in the absence of a free trade deal, prompting fierce objections from Conservative hard Brexiters and the DUP, which props up her government. That prompted May to propose a country-wide alternative in which the whole of the UK would remain in parts of the customs union after Brexit. “The EU still requires a ‘backstop to the backstop’ – effectively an insurance policy for the insurance policy. And they want this to be the Northern Ireland-only solution that they had previously proposed,” May told MPs. Raising the stakes, the prime minister said the EU’s insistence amounted to a threat to the constitution of the UK: “We have been clear that we cannot agree to anything that threatens the integrity of our United Kingdom,” she added.

The prime minister conducted a frantic round of telephone diplomacy over the weekend, speaking to Angela Merkel and Mark Rutte, the leaders of Germany and the Netherlands respectively, Downing Street confirmed.

She had also spoken to the Irish taoiseach, Leo Varadkar; the president of the European commission, Jean-Claude Juncker; and the president of the European council, Donald Tusk.

EU officials insisted the best they could offer would be “clarifications” of the withdrawal agreement, rather than reopening the text. The 585-page document states that both sides will make “best endeavours” towards negotiating a trade deal to ensure there is no need for the backstop – an all-UK customs union that would act as a guarantee against a hard border on the island of Ireland.

Earlier on Monday, a commission spokesman told reporters in Brussels: “This deal is the best and only deal possible. We will not renegotiate the deal that is on the table right now. That is very clear.

“Our position has therefore not changed and as far as we’re concerned the UK is leaving the EU on the 29 March 2019. We are prepared for all scenarios.”

That view was echoed by Varadkar, who told reporters it was not possible to renegotiate the backstop: “The reason we’ve ended up in the solution we have is because of the red lines the UK itself laid down.”

He said he would not object to the issuing of statements clarifying the meaning of the agreement, but cautioned that no clarification could go against what had already been agreed.

Guy Verhofstadt, the European parliament’s Brexit coordinator, tweeted: “I can’t follow anymore. After two years of negotiations, the Tory government wants to delay the vote. Just keep in mind that we will never let the Irish down. This delay will further aggravate the uncertainty for people and businesses. It’s time to make up their mind.”

Jeremy Corbyn described the delay as a “desperate step”.

“We have known for at least two weeks that Theresa May’s worst-of-all-worlds deal was going to be rejected by parliament because it is damaging for Britain.”

Angry Tory rebels and Labour MPs said they would try to force the government to hold a vote, even though a government source said there would be no vote on a business motion to cancel Tuesday’s vote.

“We are replacing the business with a new statement but it isn’t a motion and therefore isn’t votable,” the source said.

Steve Baker, one of the leaders of the Conservative hard Brexit European Research Group, said the postponement was “essentially a defeat” of May’s Brexit deal.

Recalling the prime minister’s general election slogan, the rebel MP added: “The terms of the WA [withdrawal agreement] were so bad that they didn’t dare put it to parliament for a vote. This isn’t the mark of a stable government or a strong plan.”

Downing Street and key ministers had repeatedly said the vote would not be delayed, in an attempt to concentrate the minds of MPs. Earlier on Monday morning, Michael Gove categorically stated the vote would go ahead.

Asked on BBC Radio 4’s Today programme if the vote was “definitely, 100%” going to happen, the environment secretary replied: “Yes”. Pressed on the point, he said: “The vote is going ahead.”