LONDON — Britain confirmed on Thursday that it would prolong its lockdown for at least three more weeks as part of efforts to keep smothering the spread of the coronavirus. But the government was far less clear about the steps it is taking that would allow it to relax restrictions later without causing another outbreak.

The widely expected extension was announced by Dominic Raab, the foreign secretary who has assumed the duties of Prime Minister Boris Johnson while he recuperates from the virus at his country residence, Chequers.

“We’re now at both a delicate and dangerous phase in this pandemic,” Mr. Raab said at a news conference. Lifting the lockdown, he said, would “risk all the progress we’ve made. Now is not the moment to give the coronavirus a second chance.”

With the extension, the lockdown will last at least until the second week of May.

Relaxing the restrictions would not only raise the risk of new outbreak, Mr. Raab said, it would also damage the economy. The government would likely be forced to impose a second lockdown, he said, which would shatter confidence.