Boris Johnson's Brexit plan has received a crushing blow just hours ahead of a critical EU summit, as the DUP said they "could not support" current proposals.

It comes as the prime minister prepares to meet with his European counterparts in Brussels, with No 10 on the brink of securing a new Brexit deal.

But in a statement released at 7am on Thursday, Mr Johnson's hopes of avoiding a Brexit extension appeared to be dashed by the DUP, who prop up the Conservatives at Westminster.

Within minutes of the announcement from the DUP leader Arlene Foster and her deputy, Nigel Dodds, the pound fell 0.5 per cent against the dollar and the euro.

"We have been involved in ongoing discussions with the government," the pair said. "As things stand, we could not support what is being suggested on customs and consent issues, and there is a lack of clarity on VAT."

But they did not entirely rule out the possibility of support a future agreement, adding: "We will continue to work with the government to try and get a sensible deal that works for Northern Ireland and protects the economic and constitutional integrity of the United Kingdom."

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The decision from the DUP effectively torpedoes the prime minister's hopes of passing any Brexit deal through the House of Commons on Saturday. He had been in close contact with Ms Foster and Mr Dodds in recent days, as he attempt to shore up their party's support.

Housing secretary Robert Jenrick told BBC Breakfast: "We always knew these negotiations were going to be difficult and we're going to go down to the line."

Asked whether the prime minister would meet with the DUP today, Mr Jenrick added: "I don't know the exact arrangements for the next few hours, but be assured the Prime Minister and his team are in intensive negotiations with all parties, including with the DUP.

"We want to provide sufficient comfort for the DUP and unionists in Northern Ireland to feel that the arrangements we would put in place with this deal are sufficient to give them comfort to support it."

DUP leader Arlene Foster (AP)

Addressing MPs on Wednesday, Stephen Barclay, the Brexit secretary, conceded that Mr Johnson will write a letter asking for an Article 50 extension if no deal is in place by 19 October.

This is despite Mr Johnson repeatedly ruling out asking for a further delay under his "do or die" commitment to leave the EU by the current Halloween deadline.

Ahead of the DUP's statement, however, Steve Baker, who chairs the European Research Group of hardline Brexiteer Conservative MPs, signalled that his group could give its backing.