“The party and Boris Johnson view the transition as purgatory,” he said, adding that Britain will swear it has no plans to ask for an extension until the last possible moment, in late May or early June. And when it does, he said, it will seek a short-term extension that would allow it to strike a trade deal as soon as possible in 2021.

The British argument for powering ahead with talks is that businesses want certainty about the new trading arrangements and that delay pushes that further into the future. Nor is it clear that more time would resolve the philosophical differences between the two sides, such as a standoff on whether Britain should sign up to some European rules to ensure fair competition.

Some believe that the economic shock of the coronavirus may embolden the British government, thinking that London’s muscle as a financial center gives it a stronger hand in negotiations.

“The City of London is going to be very important as a way for governments to raise money,” said Tony Travers, a professor of politics at the London School of Economics. “They may think the E.U. will be more concerned about upsetting things, given the scale of coronavirus, and may give them a better deal.”

Mr. Johnson also wants to avoid paying into Europe’s coffers for another year to stay part of its single market in goods and its customs territory. Perhaps worse is the political signal it might send, upsetting some Brexit hard-liners and reminding voters of the Brexit paralysis that ultimately destroyed Mr. Johnson’s predecessor as prime minister, Theresa May.

But Mr. Johnson’s refusal to budge from his Brexit mind-set also reflects a political calculation that he owes his landslide majority in Parliament to his pledge to “Get Brexit Done.”

That pro-Brexit ideology played into the government’s failure initially to join a European Union initiative to pool forces to buy ventilators and protective equipment for health workers. The government claimed it did not join the group because an email went astray, though British officials attended meetings at which the initiative was discussed.