Party sources told The Independent that their earlier statement objecting to the prime minister's Brexit plans still stands in a blow to Mr Johnson's hopes of passing the deal in parliament.
As European leaders gathered in Brussels for a critical EU summit, Mr Johnson made the shock announcement that he had struck an eleventh-hour agreement.
He said: "We've got a great new deal that takes back control - now Parliament should get Brexit done on Saturday so we can move on to other priorities like the cost of living, the NHS, violent crime and our environment."
But Mr Johnson's optimism was swiftly dashed by the DUP, whose 10 MPs act as allies to his government, and whose support is critical for his chances of winning a parliamentary majority for the blueprint.
Early on Thursday morning, DUP leader Arlene Foster and deputy Nigel Dodds issued a joint statement denouncing the prime minister's plans.
It 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.
"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.”
After Mr Johnson announced he had a deal, a DUP source said: “Read our statement. It hasn’t changed."
The Prime Minister's official spokeswoman said "today is a significant moment".
She told a Westminster briefing: "It is the best way forward for the UK. It is a deal that will take us out of the EU on October 31 and delivers for the country."
Downing Street's focus will now turn to Westminster ahead of an historic Commons sitting on Saturday, where the prime minister is expected to bring forward his Brexit deal.
Mr Johnson will face an uphill battle to pass his blueprint he cannot win round the DUP, as the party also wields significant influence over some of the more hardline Tory Brexiteers.
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