Theresa May will pull the third vote on her Brexit deal if it has no “realistic prospect” of success and will move on to asking Brussels for a long delay to article 50 on Thursday, Downing Street has said.

No 10 suggested the prime minister still wanted parliament to have another vote on her deal on Tuesday or Wednesday, but it would go ahead only if she believed the Democratic Unionist party and more Conservative MPs were on board.

May’s official spokesman confirmed talks with the DUP had resumed on Monday morning but would not say who was involved in discussions or how they were going.

If the deal did not pass this week, he said, May would keep her promise to ask the EU on Thursday for a delay to Brexit beyond June, which would involve the UK participating in European parliamentary elections costing more than £100m. He refused to say exactly how many extra months of delay the UK would request.

The question now is whether May could carry on attempting to pass her withdrawal deal until the last minute before European elections get under way. One theory circulating among Conservative MPs is that the prime minister could request a longer delay with a “get-out clause” of a quicker Brexit if her deal passes before the EU elections.

May needs to win over 75 MPs if she is to pass her deal through parliament before Thursday’s EU summit. Seventy-five Conservative MPs and 10 DUP MPs voted against her deal last time.

Twenty-three Conservative hardliners, including Boris Johnson and Dominic Raab, plus another seven Tories pushing for a second referendum have indicated they will not change their minds without the EU returning to negotiations on the Northern Ireland backstop.

This means the prime minister would need at least 20 extra Labour votes to get her deal passed. Only three Labour MPs voted with the government last time around.

No 10 rejected Johnson’s call for the prime minister to try to reopen talks with the EU, saying this was not going to happen.

Despite the difficult parliamentary maths, Jeremy Hunt, the foreign secretary, said on Monday there were “some cautious signs of encouragement” that a few Tories who had opposed the deal up to now, including Norman Lamont and Esther McVey, wanted to see it passed. “But there is a lot more work to do,” he added.

Philip Hammond, the chancellor, sounded less optimistic on Sunday. “We will only bring the deal back if we are confident that enough of our colleagues, and the DUP, are prepared to support it, so that we can get it through parliament,” he said. “We’re not going to keep presenting it if we haven’t moved the dial.”

A number of Conservative MPs believe momentum towards May’s deal will occur only if the prime minister explicitly promises to stand down to make way for a new leader to conduct the next phase of talks with the EU.

The Evening Standard reported that May’s chief Brexit adviser, Olly Robbins, could be removed from the next phase of talks to pacify hardline Brexiters, though there is little sign this would be enough to satisfy them.

Asked what the prime minister made of Tory calls for her to step aside, her spokesman said: “She is focused on conversations she is having with MPs, talks with the DUP and what we very firmly want to be in is a position where we can pass the vote this week.”