The Democratic Unionist party is unlikely to strike an agreement with Theresa May’s government to support the current withdrawal deal before Thursday’s crunch meeting with EU leaders, sources said on Monday.

With 11 days before the UK is due to leave the EU, the prime minister has been trying to convince the pro-Brexit party’s 10 MPs, who prop up her minority government, to back the Brexit deal she has agreed with the European Union.

But following a day of intense talks involving ministers and the government’s most senior civil servant, party sources said it was “extremely unlikely” that any deal would be concluded before the European council meeting.

“The reality is that practically, to get it signed off by all parties, we would have had to have a deal concluded by today to get it through before Thursday’s EU council,” said a source.

It is a blow to the government, which hoped to gain the DUP’s support before EU leaders discuss the question of delaying Brexit at their regular European council meeting.

If May’s deal had by then been approved by parliament, the government planned to request a short, technical delay of less than three months, to get the statute book ready.

Senior DUP figures including Jeffrey Donaldson and Nigel Dodds were locked in meetings on Monday with ministers including May’s de facto deputy, David Lidington, the environment secretary, Michael Gove, and the cabinet secretary, Mark Sedwill. Previous meetings have involved the chancellor, Philip Hammond, and the chief whip, Julian Smith. Meetings are pencilled in to continue over the next two days.

May has already been heavily defeated by MPs twice on her deal. On Monday, the Commons Speaker, John Bercow, ruled out May bringing her deal back for a third meaningful vote unless there was substantial change to it. Even if she is able to overcome that obstacle and put it before MPs again, the prime minister must win over 75 MPs and retain all those who previously backed her to overturn the deal’s 149-vote defeat last week.

Q&A What does the Speaker's ruling on a third meaningful vote mean? Show Hide Speaker of the House of Commons John Bercow ruled on 18 March that he would not allow parliament another vote on the same Brexit deal. Prime minister Theresa May would have to make 'substantial' changes to her government's two previous attempted to pass the withdrawal agreement deal with the EU. Bercow said: "If the government wishes to bring forward a new proposition that is neither the same nor substantially the same as that disposed of by the House on March 12, this would be entirely in order. What the government cannot legitimately do is resubmit to the house the same proposition - or substantially the same proposition - as that of last week, which was rejected by 149 votes. This ruling should not be regarded as my last word on the subject. It is simply meant to indicate the test which the government must meet in order for me to rule that a third meaningful vote can legitimately be held in this parliamentary session." Photograph: Reuters Tv/X00514

The DUP rejected the concessions won by the prime minister in Strasbourg last week after the attorney general, Geoffrey Cox, said they would not prevent the UK being held in the Northern Ireland backstop permanently.

Hammond said the main emphasis was now on reassuring the DUP that new barriers would not be allowed to develop between Northern Ireland and Great Britain if the backstop was invoked.

Asked on Sunday if extra funding for Northern Ireland could be involved, as it was in clinching the DUP’s support for May’s minority government two years ago, the chancellor said: “We are coming up to a spending review and we will have to look at all budgets, including devolved block-grant budgets, in that spending review; of course we will.”

Arlene Foster, the DUP’s leader, has not taken part in Monday’s meetings and has chosen to stay in Belfast. Asked about the talks with ministers, a DUP spokesman said the party was concentrating on preserving the union and delivering on the referendum result.