Boris Johnson has said the coronavirus lockdown cannot be eased yet as the UK is at the point of “maximum risk” in its battle with the outbreak.

In his first public appearance since he returned to work, the prime minister said there were signs the country was “turning the tide” against the virus but warned the public that it was too early to lift the restrictions.

Mr Johnson said that it was too early to spell out details of how or when the lockdown would be lifted, but indicated that this would happen “in the coming days” and that the removal of restrictions will be “gradual”.

After his own battle with the virus, Mr Johnson described Covid-19 as “an unexpected and invisible mugger” and said that now was the moment “we have begun together to wrestle it to the floor”.

Speaking outside Downing Street, he said: “And so it follows that this is the moment of opportunity, this is the moment when we can press home our advantage, it is also the moment of maximum risk.

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“I know there will be many people looking at our apparent success, and beginning to wonder whether now is the time to go easy on those social-distancing measures.

“And I know how hard and stressful it has been to give up, even temporarily, those ancient and basic freedoms, not seeing friends, not seeing loved ones, working from home, managing the kids, worrying about your job and your firm.”

Addressing businesses – and members of his own party, Mr Johnson said he understood their impatience and anxiety but insisted the country must avoid a second spike in coronavirus cases, when it would once again have to “slam on the brakes across the whole country”.

“I want to get this economy moving as fast as I can but I refuse to throw away all the effort, and the sacrifice of the British people and to risk a second major outbreak and huge loss of life and the overwhelming of the NHS,” he said.

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He said the outbreak was the “biggest single challenge this country has faced since the war” but there had been fewer hospital admissions, fewer Covid patients in ICU and “real signs now that we are passing through the peak”

The prime minister added: “And thanks to your forbearance, your good sense, your altruism, your spirit of community, thanks to our collective national resolve, we are on the brink of achieving that first clear mission to prevent our NHS from being overwhelmed in a way that tragically we have seen elsewhere.

“And that is how and why we are now beginning to turn the tide.”

Mr Johnson said preparations have been under way “for weeks” in government to find a path out of the lockdown.

Restrictions can be eased only “when we’re sure that this first phase is over” and that the government’s five tests – falling rates of deaths and infections, protection of the NHS, sorting out testing and PPE provision and avoiding a second peak of cases – have been met, he said.

“Then that will be the time to move on to the second phase in which we continue to suppress the disease and keep the reproduction rate – the R rate – down, but begin gradually to refine the economic and social restrictions, and one by one to fire up the engines of this vast UK economy,” said Mr Johnson.

“In that process, difficult judgements will be made. And we simply cannot spell out now how fast or slow – or even when – those changes will be made, though clearly the government will be saying much more about this in the coming days.”

Mr Johnson vowed to take decisions on lifting lockdown “with the maximum possible transparency”, in discussion with opposition parties and business.

It comes after the prime minister was out of action for three weeks with Covid-19, including a three-night stay in intensive care where he battled the virus.