LONDON (REUTERS) - Britain will not be able to relax its stringent lockdown rules until the end of May, a leading government adviser said on Saturday (April 4), as the death toll rose to 4,313

The government has put Britain into a widespread shutdown, closing pubs, restaurants and nearly all shops, while ordering people to stay home unless absolutely essential to venture out.

The order is designed to curb the spread of Covid-19 in the country, which has almost 42,000 confirmed cases.

But some experts have started to question whether the shuttering of the economy will cost more lives in the long run.

Prof Neil Ferguson, a leading professor of mathematical biology at Imperial College London, said work was under way to establish how more relaxed rules could be introduced in time.

"We want to move to a situation where at least by the end of May that we're able to substitute some less intensive measures, more based on technology and testing, for the complete lockdown we have now," he told BBC Radio.

"There is a great deal of work under way to look at how we can substitute some of the very intense social distancing currently in place with a regime more based on intensive testing, very rapid access to testing, contact tracing of contracts. But in order to substitute that regime for what we're doing now, we need to get case numbers down."

Britain's death toll from the coronavirus rose by 20 per cent to 4,313 by Friday afternoon with 708 new fatalities recorded, the health ministry said. That compared to a 23 per cent rise on Thursday.

The peak of new cases could come within a week or 10 days, Prof Ferguson said, but adherence to the strict rules will determine how quickly the rate of infections decline after that.

"It is quite finely balanced at the current time," he said, adding that Britain could have quite high levels of infection for "weeks and weeks" if people start to socialise.

Britain initially took a restrained approach to the outbreak but Prime Minister Boris Johnson changed tack and imposed stringent social-distancing measures after Prof Ferguson's modelling showed a quarter of a million people in the country could die.

The response has since been hampered by a lack of ventilators and an inability to carry out mass testing to determine whether the public, and particularly health workers, have built up an immunity.

Mr Johnson, who has been in self-isolation after testing positive for the novel coronavirus, has invited opposition party leaders to a briefing next week with medical advisers, including the new leader of the Labour Party, Keir Starmer.

"As party leaders we have a duty to work together at this moment of national emergency," he said.

Some are questioning the long-term strategy.

A second senior government adviser, chief pandemic modeller Graham Medley, told The Times newspaper on Saturday he feared Britain had painted itself in a corner with no clear exit from a strategy that will damage the economic and mental well-being of much of the population.

Almost one million people have applied for welfare benefits in just two weeks in Britain, according to official data that shows the economy is set for a depression that could be worse than the slump in the 1930s.

"If we carry on with lockdown it buys us more time, we can get more thought put into it, but it doesn't resolve anything - it's a placeholder," Mr Medley told the Times newspaper. "We've kind of painted ourselves into a corner, because then the question will be, what do we do now? In broad terms are we going to continue to harm children to protect vulnerable people, or not?"

Health Minister Matt Hancock has set a goal of 100,000 tests per day by the end of this month, a tenfold increase that industry leaders have questioned due to shortages of equipment. It is also considering immunity certificates.

Separately the government said it would free prisoners who were deemed to be low risk and were within weeks of release.