Britain and the European Union on Monday struck a landmark deal aimed at boosting security cooperation while removing some of the barriers to trade introduced by Brexit.

The agreement is designed to help the two sides work more closely together as the United States has signaled it is reducing its commitment to European security.

But it also underscores the ambition of British Prime Minister Keir Starmer for a “reset” of ties with the 27-nation bloc, almost nine years after Britons voted by a narrow margin to leave the European Union.

Nick Thomas-Symonds, a senior British minister who has worked closely on the negotiations, described Monday as a “historic day,” writing on social media that the government “has secured a new agreement with the E.U.”

He said it was good for jobs, would reduce bills and strengthen borders. It also puts “Britain back on the world stage, with a government in the service of working people, he added.”

However, the agreement with the European Union — Britain’s biggest trading partner — is politically contentious at home and has involved months of difficult negotiations.

One of the most delicate issues in the trade talks was an extension of the right of European trawlers to fish in British territorial waters, in exchange for reduced trade barriers for British food products entering the E.U.’s giant single economic zone.

After discussions that went to the last minute, the two sides agreed that they would allow European boats access to U.K. fishing waters until June 30, 2038, according to a European diplomat who spoke on condition of anonymity to preview the decision.

That was not the indefinite extension that some in Europe had sought, but is a far longer window than the British had initially suggested. It was quickly seized on by Britain’s right-wing, pro-Brexit tabloids, with one calling it a “surrender.”

Given Britain’s status as one of Europe’s two main military powers, the European Union has long wanted a security pact with Britain. That idea, however, was rejected by the government led by the former prime minister, Boris Johnson, when he negotiated Britain’s exit from the bloc.

The war in Ukraine and President Trump’s approach to European security have changed the climate and increased the sense of urgency for cooperation. At the same time, Britain is pushing for some of its companies to participate in a 150 billion euro loan program to fund joint defense procurement.

This is a developing story. Please check back for updates.