U.K. and E.U. Reach Post-Brexit Trade Deal on Northern Ireland: Live Updates

But the deal is a major risk for Mr. Sunak, opening him up to a backlash from pro-Brexit hard-liners in his Conservative Party and the Democratic Unionist Party in Northern Ireland, which has campaigned to rewrite the post-Brexit trade rules, rather than simply modify them, as Mr. Sunak and Ms. von der Leyen have done.

Northern Ireland’s unique status — a part of the United Kingdom but sharing a land border with Ireland, which is a member of the European Union and its single market — has made its current trade terms a totemic issue for Brexiteers and unionists, the largely Protestant part of the territory’s population that wants it to remain in the United Kingdom.

The rules were designed to avoid checks at the land border, which would be unacceptable for Ireland and for Northern Ireland’s nationalists, the largely Catholic part of the population that wants the territory to be reunited with Ireland.

For Mr. Sunak, who came to power last October and is trailing the opposition Labour Party in the polls, the deal is a litmus test for his young government. A negative reaction could embolden one of his predecessors, Boris Johnson, who was ousted last year but who may harbor ambitions for a comeback.

Part of the problem is that Mr. Sunak negotiated the deal with Ms. von der Leyen under a veil of secrecy. This has heightened suspicions among unionists and Brexit enthusiasts, who oppose any deal that applies E.U. trade rules in Northern Ireland, and does not treat it the same as the other countries of the United Kingdom.

Mr. Sunak has also been noncommittal about whether Parliament would be allowed to approve it. On Sunday, Mr. Sunak’s deputy prime minister, Dominic Raab, declined to confirm that lawmakers in Westminster would get to vote on the deal.

“Parliament will find a way to have its say,” Mr. Raab said on Sky News, without explaining what that meant.

The outline agreement would revamp a document known as the Northern Ireland Protocol, which was created to avoid the need for customs controls on goods crossing the politically sensitive border between Northern Ireland and Ireland, and came into force in early 2021. Under the protocol, Northern Ireland stayed within parts of Europe’s single market, abiding by its economic rule book.

But the protocol also created a different trade barrier that required checks on goods moving from Britain to Northern Ireland. That infuriated much of Northern Ireland’s unionist community, which feared that it drove a wedge between them and the rest of the United Kingdom.

To protest the protocol, the Democratic Unionist Party has been boycotting Northern Ireland’s Assembly and devolved government. To ensure that power is shared between unionists and nationalists, the system shuts down unless the largest parties on both sides agree to participate.

Whether the new agreement is robust enough to persuade the party to go back into Northern Ireland’s government will be one test of the success of Mr. Sunak’s negotiation.

In an interview with the Sunday Times, Mr. Sunak said, “I want to correct the democratic deficit because sovereignty is really important, and that’s why the idea that the E.U. can impose laws on Northern Ireland without them having a say isn’t acceptable.”

Another key reaction will be from Conservative Brexit supporters, including Mr. Johnson. He agreed to the protocol during his time in office, but later angered E.U. nations by introducing legislation intended to give the British government power to override some of it.

That bill is currently being examined by the House of Lords, the unelected revising chamber of the British Parliament, but Mr. Sunak has agreed to scrap it as a price for the concessions made by Brussels. Mr. Johnson, according to British news reports, has told allies that abandoning the bill would be a “great mistake.”

Some observers think Mr. Johnson is preparing to destabilize Mr. Sunak’s leadership and perhaps to try to oust him if the Conservative Party performs poorly in local municipality elections scheduled for May.

“He wants to bring down Rishi Sunak, and he will use any instrument to do it,” George Osborne, a Conservative and a former chancellor of the Exchequer, told Channel 4 in late February. “And if the Northern Ireland negotiations are that instrument, he will pick it up and hit Mr. Sunak over the head with it.”