But that Britain is the first country in the world to conclude a deal with the United States after Mr. Trump imposed an across-the-board tariff of 10 percent on dozens of American trading partners seemed to affirm the “special relationship” often invoked by British and American diplomats.

In a post on Thursday morning, Mr. Trump said the agreement was a “full and comprehensive one that will cement the relationship between the United States and the United Kingdom for many years to come.”

Britain, even under a center-left Labour government, has escaped the harsh criticism that Mr. Trump has trained on the European Union and China, both of which he accuses of “ripping off” the United States in their trade relationships.

That partly reflects economics: Britain and the United States have reasonably balanced trade flows. Britain runs either an $89 billion trade surplus or a $14.5 billion deficit with the United States, depending on whether one cites British or American statistics.

The difference rests in part on how the two sides treat offshore financial centers like Jersey and Guernsey, which are semiautonomous. Crucially, trade in goods, with which Mr. Trump is most fixated, is relatively in balance.

But it also reflects Mr. Starmer’s tireless diplomatic outreach to Mr. Trump. In February, the prime minister turned up in the Oval Office with a letter from King Charles III, inviting Mr. Trump to make a rare second state visit to Britain. Mr. Starmer has stayed in touch with Mr. Trump since, speaking to him regularly by phone about pet issues like the president’s Trump Turnberry golf club in Scotland.

Mr. Starmer has steadfastly refused to criticize Mr. Trump, either on trade or his statements about Ukraine and its war with Russia. The prime minister has tried to act as a bridge between Europe and the United States, coaching President Volodymyr Zelensky of Ukraine on how to mend his rift with Mr. Trump after the two men clashed in an Oval Office several days after Mr. Starmer’s much friendlier visit.

Britain had other advantages in dealing with the United States. Its new ambassador to Washington, Peter Mandelson, had been a trade commissioner at the European Union and is steeped in the mechanics of trade negotiations.

But the British had to navigate the foibles of Trump administration, in which trade talks exist on two distinct levels: the substantive negotiations between teams at the United States Trade Representative and Britain’s trade ministry, and the whims of Mr. Trump, who seemed eager to announce a showcase deal but whose blessing was required before any agreement could be clinched.

At home, the deal could help buoy Britain’s sputtering confidence. Mr. Starmer has struggled with faltering economic growth, as well as straitened public finances, which have led his government into a series of unpopular moves, like curbing a subsidy to help older people pay their winter heating bills.

But the deal could also present Mr. Starmer with some hurdles, including to his efforts to reset trade relations with the European Union after Brexit. Some analysts have suggested that Britain will have to choose whether to align with the United States or Europe — something British officials dismiss as a false choice.

As the terms of the deal become clear, British farmers will be watching to see if their agricultural goods, notably chicken and beef, are protected from American imports. The government had drawn a red line against allowing imports of American “chlorinated” chicken or chemically treated beef.

Mr. Starmer was expected to address the deal later on Thursday. Much of his day will be given over to commemorating the 80th anniversary of the Allied victory over Nazi Germany in World War II. That history provides a fitting backdrop for a trade agreement that Britain will no doubt present as a symbol of its enduring ties to the United States.

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“Because of our long time history and allegiance together, it is a great honor to have the United Kingdom as our FIRST announcement,” he wrote. “Many other deals, which are in serious stages of negotiation, to follow!”

Mr. Trump is expected to announce the deal from the Oval Office later Thursday.

The president had not specified which nation would part of the deal in his post late Wednesday night. On Thursday, a senior British official confirmed that a deal with the United States had been reached.

The British official, who spoke on the condition of anonymity because of the sensitivity of the issue, did not offer details, beyond saying that the deal would be good for both Britain and the United States.

The agreement would be the first deal announced since Mr. Trump imposed stiff tariffs on dozens of America’s trading partners. He later paused those temporarily in order to allow other nations to reach agreements with the United States.

Details of the agreement were not immediately clear. Both nations have discussed lowering British tariffs on U.S. cars and farm goods, as well as removing British taxes on U.S. technology companies. It also was not clear whether the agreement had actually been finalized.

Timothy C. Brightbill, an international trade attorney at Wiley Rein, said the announcement would probably be “just an agreement to start the negotiations, identifying a framework of issues to be discussed in the coming months.”

“We suspect that tariff rates, nontariff barriers and digital trade are all on the list — and there are difficult issues to address on all of these,” he added.

The Trump administration has been trying to cajole other countries into reaching quick trade deals with the United States. The president imposed punishing tariffs on dozens of its trading partners on April 2, but quickly backtracked after panic ensued in the bond market. Mr. Trump paused most of those tariffs for 90 days so that the United States could negotiate trade deals with other nations.

But he has left a 10 percent global tariff in place, including on Britain. Unlike other countries, Britain was not subjected to higher “reciprocal” tariffs, because it buys more from the United States than it sells to it.

Britain is also subject to a 25 percent tariff that Mr. Trump has placed on foreign steel, aluminum and automobiles, levies that British officials have been pushing their U.S. counterparts to lift.

Mr. Trump’s interest in striking a trade deal with Britain dates back to his first term, when his advisers negotiated with the country but didn’t finalize an agreement. British officials have also been eyeing a trade agreement with the United States since Brexit, as a way to offset weaker relations with Europe. In the Biden administration, British officials continued to push for a deal with the United States but made little progress.

For Britain’s prime minister, Keir Starmer, the trade deal would offer vindication for his assiduous cultivation of Mr. Trump. During his visit to the Oval Office in February, Mr. Starmer turned up with an invitation from King Charles III for the president to make a rare second state visit to Britain.

The Trump administration appears to be nearing deals with India and Israel, and is continuing to negotiate with South Korea, Japan, Vietnam and other nations. Still, Mr. Trump once again displayed his unpredictable approach to economic policy on Tuesday when he downplayed the prospect of trade deals, saying other countries needed such agreements more than the United States.

“Everyone says ‘When, when, when are you going to sign deals?’” Mr. Trump said, at one point motioning toward Howard Lutnick, his commerce secretary. “We don’t have to sign deals. We could sign 25 deals right now, Howard, if we wanted to. We don’t have to sign deals. They have to sign deals with us.”

Trade experts have said that Mr. Trump may be intending to announce far more limited deals than traditional trade agreements, which cover most trade between countries and require congressional approval. Historically, free-trade agreements have taken the United States more than a year to negotiate.

In his first term, Mr. Trump renegotiated several U.S. trade agreements, including a free-trade agreement with South Korea and NAFTA. But he also signed a series of more limited “mini-deals” with countries in which they reduced tariffs on a few kinds of goods or agreed to talk about a few sectors.

British officials have also been negotiating with the European Union, and on Tuesday agreed to a trade deal with India. The India deal would lower tariffs between the countries and secure more access for British firms to India’s insurance and banking sectors, among other changes. The announcement followed nearly three years of negotiations.

Mark Landler contributed reporting.