Boris Johnson promised to set out next week how schools and workplaces could safely reopen once lockdown restrictions were eased, as he claimed the UK was past the peak of the coronavirus outbreak.

Despite announcing an increase of 674 in the death toll of the virus, taking it to a total of 26,771, the prime minister suggested the worst was over.

“I can confirm today for the first time that we are past the peak of this disease. We are past the peak and we are on the downward slope,” he said, three days after returning to work as he recovers from Covid-19 himself.

Presenting his first Downing Street press conference since being discharged from hospital, the prime minister at times appeared breathless.

He robustly defended his government’s handling of the outbreak, after a slew of criticism during his absence from the political frontline. “We’re learning lessons every day – but I do think that, broadly speaking, we did the right thing at the right time,” he said.

Responding to the argument that a more stringent lockdown should have been imposed sooner, he said: “Don’t forget, it’s a very very demanding thing to ask a population to do – very tough – and so I think it was completely right to make our period of lockdown coincide as far as possible with the peak of the epidemic.”

Johnson announced that the government would next week publish what he called a “comprehensive plan” setting out “how we can continue to suppress the disease and at the same time restart the economy”.

However, he stressed any changes would only be enacted once the government was confident the outbreak was under control.

“What you are going to get next week is really a road map, a menu of options. The dates and times of each individual measure will be very much driven by where we are in the epidemic, what the data is really saying and we are getting in a lot more data every day now and in the course of the next few days.”

The Labour leader, Keir Starmer, welcomed the prime minister’s statement, after repeatedly calling on the government to set out its “exit strategy” – though Johnson declined to use that precise phrase.

“I’ve been calling on the prime minister to have a plan for the next stage and exit strategy,” Starmer said. “We’ve been pushing hard on that in the last week or two. The prime minister has now said he’s going to have a plan next week. So I think that shows that we were right to challenge on it, and I’m pleased that we’re going to see a plan. We’ll look at it when we see it, but it’s a step in the right direction.”

The government’s five tests for relaxing lockdown restrictions include the availability of adequate PPE and testing, and a sustained fall in the death rate.

But the prime minister laid particular emphasis on R, the reproduction number for the virus, measured as the average number of people each sufferer infects.

The target is to keep the R rate of transmission below 1, but even small reductions in the rate can help avoid thousands of new infections a day. At the moment, the government’s scientific advisers believe it is between 0.6 and 0.9.

England’s chief medical officer, Chris Whitty, urged caution, saying: “As soon as R goes over one, then you restart exponential growth – it may be slow if it is just over one, it may be a lot faster if it goes a lot above one – but exponential growth restarts and, sooner or later – and the higher it is, the sooner it is – the NHS will go back to the risk of being overwhelmed and the number of cases will go up.”

One source close to Downing Street said the government was also reluctant to lift the lockdown until the number of new daily infections was lower than around 5,000 per day – the measure which was used by Germany to determine when to ease social distancing.

However, the UK is quite some way off that goal. The official figures show at least 6,000 people tested positive on Wednesday, but the real number is likely to be much higher than that because many cases in the community are not being tested.

The Scottish first minister, Nicola Sturgeon, told the Scottish public on Wednesday it was highly unlikely that there would be a loosening of lockdown restrictions next week.

“It is likely by the time we get to this time next week, which is the 7 May review day, it will not be possible in any meaningful way to lift these restrictions,” she said.

“We have worked very hard to bring down the transmission rate of the virus and we are definitely seeing results from those efforts. But our progress against the virus is too fragile for us to let up.”

In Whitehall, ministers have been scrambling to draw up plans for the next phase of the crisis, which Johnson’s deputy, Dominic Raab, has described as a “new normal”, rather than a return to the pre-lockdown life.

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Johnson said for the first time on Thursdaythat this could include the public being advised to wear face masks.

Asked about the issue, the prime minister said: “I do think face masks will be useful, both for epidemiological reasons, and to give people the confidence that it is safe to go back to work.”

His remarks contradicted the stance of some cabinet colleagues. The health secretary, Matt Hancock, said earlier this week there was only “weak science” supporting the use of face masks. The Cabinet Office minister, Michael Gove, warned on Wednesday they might tempt people to behave in a “cavalier” manner.

The mayor of London, Sadiq Khan, welcomed the government’s apparent change of heart, saying: “I am pleased that the prime minister has said that non-medical face coverings will play an important role as we eventually look to lift lockdown restrictions. This is something I and others have been pushing the government to change the guidance on.”

Downing Street believes more people could safely return to work even before the lockdown is formally lifted, if distancing measures are complied with.

Some manufacturers, including the car maker Rolls-Royce, have restarted production lines – and the DIY chain B&Q announced on Wednesday that it would reopen its stores.

The chancellor, Rishi Sunak, has been keen to see more of the economy reopen as soon as it is safe to do so. The independent Office for Budget Responsibility has estimated that the unprecedented measures introduced to help firms and households weather the crisis will cost £104bn in this financial year.

Asked whether meeting the costs would mean a fresh round of austerity when the outbreak was over, Johnson insisted it would not, saying: “That will certainly not be part of our approach.”

“I think the economy will want to bounce back strongly. I think that this government will want to encourage that bounceback in all kinds of ways,” he said.

In preparation for an easing of lockdown restrictions, the business secretary, Alok Sharma, is drawing up guidance for safe working in different types of setting. Business groups and unions are expecting to see a first draft of this weekend.

Q&A What are the UK government's 'five tests' for ending lockdown restrictions? Show Hide In April the UK government set out these five tests it said had to be met before they would consider easing coronavirus lockdown restrictions: The NHS has sufficient capacity to provide critical care and specialist treatment right across the UK

A sustained and consistent fall in daily deaths from Coronavirus

Reliable data to show that the rate of infection is decreasing to manageable levels across the board

Operational challenges including testing and personal protective equipment (PPE) are in hand with supply able to meet future demand

Confident that any adjustments to the current measures will not risk a second peak of infections that overwhelms the NHS

The education secretary, Gavin Williamson, has been examining plans for what he has called a “phased” reopening of schools.

Testing for coronavirus has increased significantly in recent days as eligibility has been extended to new groups, with 81,611 tests carried out on Wednesday, according to the Department of Health and Social Care, although only 54,575 individuals were tested because of the number of repeated tests on the same people. Testing capacity stood at around 90,000.

It will not be clear until Friday afternoon whether Hancock has met his self-imposed target of 100,000 tests a day by the end of April.