Britain will not be able to relax its stringent lockdown rules until the end of May, a leading government adviser said on Saturday, warning that first the spread of coronavirus must slow and intense testing must be introduced.

Neil Ferguson, a leading professor of mathematical biology at Imperial College London, said work was underway 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 underway 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."

The government has put Britain into a widespread lockdown, closing pubs, restaurants and nearly all shops, while banning social gatherings and ordering people to stay at home unless it is absolutely essential to venture out.

By Friday, Britain's death toll from the global pandemic had risen to 3,605, with 38,168 people having tested positive for the highly infectious respiratory illness known as COVID-19.

A second senior government adviser, the chief pandemic modeler 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.