Three more weeks of lockdown for Britain. Britain confirmed on Thursday that it would prolong its coronavirus lockdown for at least three more weeks. But the government shed little light on how it might eventually relax restrictions without causing another surge in infections. The widely expected extension was announced by Dominic Raab, the foreign secretary. Mr. Raab has assumed the duties of Prime Minister Boris Johnson, who is recuperating 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,” he said. Relaxing the restrictions would not only raise the risk of new outbreak, Mr. Raab said, it would also damage the economy. The government would then probably be forced to impose a second lockdown, he said, which might shatter confidence. The restrictions will now last at least until the second week of May. Mr. Raab set out five prerequisites for easing restrictions. They included a “sustained and consistent fall in the daily death rates,” confidence that hospitals could cope with the flow of patients, more capacity for testing, more protective equipment, and a judgment, made with the advice of government health experts, that there would not be a second wave of infections. With 861 new deaths announced on Thursday — 100 more than the day before — along with complaints about a lack of masks and gloves, and a major shortfall in testing, Britain appears far from meeting three of those prerequisites. Only the hospitals, with a small decline in the number of coronavirus patients and a growing number of beds, are a bright spot.