Boris Johnson’s hopes of a Brexit deal before tomorrow’s crunch EU summit are fading as new “hurdles” emerge, The Independent understands.

Talks resumed in Brussels this morning, but with increasing gloom that an agreement will be ready for EU leaders to rubberstamp it this week.

It is unclear what the new obstacles are, but the bleaker mood follows a warning from the Democratic Unionist Party that it will not accept the apparent concession of a customs border in the Irish Sea.

Arlene Foster warned the prime minister, after talks in No 10 last night, that the DUP would “stick with our principles” that Northern Ireland “must remain” in the UK’s customs territory.

Mr Johnson has made a series of concession in recent days in order to secure a deal before the two-day EU summit gets underway in Brussels on Thursday, EU sources have said.

But it appears he will not strike an agreement without the backing of the DUP, which – with Brexiteer Tories following its lead – has the power to sink it back at Westminster.

A government source told The Independent there were still “hurdles to overcome” that Downing Street had hoped to avoid, with the clock ticking.

A failure to agree a deal would see Mr Johnson forced to break his promise and seek an extension to Article 50 to delay Brexit, a deadline that kicks in at 11pm on Saturday.

The lengthening odds on a deal came after No 10 had insisted “constructive talks “were continuing in Brussels, ahead of Mr Johnson briefing the cabinet at 2.30pm on Wednesday afternoon.

Boris Johnson and Brexit merchandise for sale at the Tory conference Show all 10 1 /10 Boris Johnson and Brexit merchandise for sale at the Tory conference Boris Johnson and Brexit merchandise for sale at the Tory conference A Boris Johnson doll for sale at the Conservative Party Conference in Manchester on 29 September PA Boris Johnson and Brexit merchandise for sale at the Tory conference Mugs with Boris Johnson's portrait for sale at the Conservative Party Conference in Manchester on 29 September AP Boris Johnson and Brexit merchandise for sale at the Tory conference A Boris Johnson themed T-shirt for sale at the Conservative Party Conference in Manchester on 29 September AFP/Getty Boris Johnson and Brexit merchandise for sale at the Tory conference Boris Johnson and other Conservative themed cards for sale at the Conservative Party Conference in Manchester on 29 September AP Boris Johnson and Brexit merchandise for sale at the Tory conference Boris Johnson posters for sale at the Conservative Party Conference in Manchester on 29 September EPA Boris Johnson and Brexit merchandise for sale at the Tory conference Copies of The Conservative newspaper are ready to be distributed during the Conservative Party conference on 30 September Getty Boris Johnson and Brexit merchandise for sale at the Tory conference "GET BREXIT DONE" mugs alongside other Conservative merchandise at the party conference in Manchester on 29 September PA Boris Johnson and Brexit merchandise for sale at the Tory conference Assorted Conservative merchandise for sale at the party conference in Manchester on 29 September PA Boris Johnson and Brexit merchandise for sale at the Tory conference Books about Jacob Rees-Mogg and Jeremy Corbyn for sale at the Conservative Party conference in Manchester on 29 September Getty Boris Johnson and Brexit merchandise for sale at the Tory conference A variety of books for sale at the Conservative Party conference in Manchester on 29 September Getty

But just as crucial are the domestic talks with the hardline European Research Group of Tory backbenchers and the DUP.

Ms Foster told the Irish TV channel RTE that she would not accept a border in the Irish Sea – something Theresa May had argued was unacceptable to any UK prime minister.

“We do want to get a deal, but it has to be a deal that respects the constitutional and economic place of Northern Ireland within the United Kingdom and there has to be consent,” she said.

“There has to be consent which is in accordance with the Belfast Agreement, in other words there has to be consent from the nationalist community and the Unionist community.”

Owen Paterson, a leading ERG member told The Sun it would be “unacceptable” for custom checks to be carried out on goods travelling between Northern Ireland and Great Britain.

But Mark Francois, the ERG’s deputy chairman, said it had yet to see details of the proposed deal and would meet early on Saturday to decide whether to back it.