BRUSSELS — Britain’s frantic efforts to negotiate a Brexit agreement with the European Union hit a last-minute snag on Thursday morning, after Northern Ireland’s Democratic Unionist Party said it could not support the deal “as things stand.”
The statement, hours before Prime Minister Boris Johnson of Britain was to present the deal to European leaders at a summit meeting in Brussels, suggested that British domestic politics once again threatened to torpedo a complex negotiation in the 11th hour.
It was not clear whether the Northern Ireland party simply wanted to make a show of holding out for its position before ultimately acquiescing — or whether Mr. Johnson faced a serious rebellion from the skeptics in his ranks.
But the sudden setback rattled the financial markets, with the British pound falling to $1.27 and 1.15 euros on the news, after having surged earlier this week on optimism that a deal was finally in sight.
The Democratic Unionists, who have proved to be a pivotal blocking force in previous attempts to negotiate an agreement to extricate Britain from the bloc, said they were troubled by elements of the deal on how to handle Northern Ireland in a post-Brexit world.
“As things stand,” the party said in a statement issued on Thursday, “we could not support what is being suggested on a customs and consent issues, and there is a lack of clarity on VAT.”
The party said it would continue working with the government on an acceptable agreement.
Mr. Johnson has consulted closely the Democratic Unionists and other skeptical elements of his Conservative Party-led coalition as a deal has taken shape. On Wednesday, optimism had grown amid signs in Brussels that the deadlock over Britain’s planned departure from the bloc could be on the verge of breaking.
Essentially, the proposed agreement would leave Northern Ireland aligned with European Union laws and regulations on most trade issues, even as it moved out of the European single market and into a customs union with Britain.
Under the proposed terms, the bloc would still conduct customs checks on some goods flowing from Britain to Northern Ireland if those goods were ultimately destined for the European Union.
There would be a complex series of rules on tariffs and value-added tax payments to compensate for differences in tariff rates between the European and British customs unions, though negotiators were struggling late on Tuesday to resolve the issue of how to rebate value-added tax payments.
The arrangement would also be subject to consent by the Northern Ireland Assembly, but in a way that would prevent the Democratic Unionists, who have opposed previous such proposals, from simply vetoing it at the first possible opportunity.
The Democratic Unionists are crucial to Mr. Johnson’s effort to win a majority for the deal in Parliament. Their opposition to similar previous versions of a Brexit agreement forced Mr. Johnson’s predecessor, Theresa May, to overhaul that agreement to place all of Britain in the European customs union for a period of time.
Mrs. May’s deal was nevertheless soundly defeated in Parliament three times.
Mr. Johnson was seen as having a better chance of cobbling together a majority, in part because he was a vocal supporter of Brexit before the 2016 referendum and thus has greater credibility with euroskeptic elements of the Conservative coalition.
As British negotiators were huddling with their European counterparts in Brussels, Mr. Johnson met with a parade of skeptics in 10 Downing Street. His hope is to win approval of the deal in Brussels by Friday, and then put it to a vote in a special session of the House of Commons on Saturday.
Mr. Johnson has vowed to leave the European Union, with or without a deal, by Oct. 31, and his negotiators have labored to seal an agreement by this week so that the prime minister is not force to ask Brussels for an extension.
European negotiators have used that tight timetable as leverage to push Mr. Johnson on the issue of Northern Ireland. The deal taking shape is not all that different from the one that the European Union proposed to Mrs. May — and which she concluded she could not accept because of opposition from the Democratic Unionists.