LONDON — British Prime Minister Boris Johnson on Monday resisted calls to loosen the country's lockdown as he resumed work three weeks after being admitted to an intensive care unit with a serious case of the coronavirus.

Johnson — the first world leader to fall victim to the disease — urged caution and gave no clue as to how or when thousands of schools and businesses might return to normal, calling this the time of "maximum risk" of a second wave of infections.

Several neighboring European nations have either published plans or begun lifting elements of their national lockdowns. Social distancing measures will be relaxed in Italy — Europe's worst-affected country — on May 4, with restaurants reopening next month. Johnson's government has also come under growing pressure from some fellow Conservative Party members and business leaders to ease restrictions and help the ailing economy.

"I'm sorry I've been away from my desk for much longer than I would have liked," said Johnson, 55, referring to his period of recuperation.

"If this virus were a physical assailant, an invisible and unexpected mugger, which I can tell you from personal experience it is, then this is the moment when we have begun together to wrestle it to the floor," he added in a speech outside No. 10 Downing Street.

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Johnson, who was initially reluctant to impose strict lockdown measures, was speaking three weeks after he was placed in an intensive care unit at a London hospital. He was discharged after six days and some of his duties were delegated to Foreign Secretary Dominic Raab.

The U.K. has registered 20,000 deaths from the coronavirus outbreak, according to data from Johns Hopkins University, giving it the fourth-highest death toll in the world, despite being 22nd in global population.

In the week to April 10, some 1,000 people died in nursing homes in the U.K., according to the Office for National Statistics, suggesting a vast uncounted number of deaths among older and vulnerable Britons outside hospitals.

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Business leaders have called for clarity and a faster relaxation of the lockdown to avoid an economic disaster. The EY Item Club, a London-based forecasting group, said Monday it expected the U.K. economy to shrink by a record 6.8 percent this year in a deep, short recession.

Steve Morgan, a property industry leader and Conservative Party donor through his property business Bridgemere Group, told BBC radio on Monday, "We need to get back to work, the country can't afford to continue in a lockdown situation."

Sir Graham Brady, an influential Conservative lawmaker, on Monday called for "inconsistencies" in the social distancing measures to be addressed and asked why they couldn't be removed more quickly.

Under increasing pressure, Johnson held firm.

"I know there will be people looking at our apparent success and begin to wonder whether now is the time to go easy on those social distancing measures," he said.

"And yet we must also recognize the risk of a second spike, of losing control of that virus and the letting the reproduction rate going back over one. Because that would not only be a new wave of death and disease but an economic disaster, and we would be forced once again to slam on the brakes on the while country and the whole economy."

Johnson's ruling Conservative Party also faces pressure from the opposition Labour Party, under its new leader, Sir Keir Starmer, which is pressing for making public a plan to lift the lockdown.

"We cannot spell out how fast or how slow and even when those changes will be made," he said. "Although clearly the government will be saying more about about this in the coming days."