Credit: CC0 Public Domain

The UK government's chief scientific adviser on Tuesday said it was "reasonable" to estimate that 55,000 people could have COVID-19, based on the expected death rate of one in 1,000 cases.

Asked at a parliamentary health committee if there were "potentially 55,000 cases" based on that ratio, Patrick Vallance said that was "a reasonable ballpark way of looking at it".

But he added the modelling was "not more accurate than that".

Britain had been criticised for its light touch approach to containing the , although on Monday it laid out tougher new recommendations to halt the spread.

There were 407 new cases in Britain in the last 24 hours, taking the of confirmed patients with the virus to 1,950, the Department of Health said. Fifty-six people have died.

The UK approach has been designed to ease pressure on its already overburdened state-run health system by flattening the curve of the outbreak, and spreading cases over a longer period.

Vallance said it was a "daily, changing, unique situation, where we're learning as we go along".

But he said of fatalities: "If we can get numbers down to 20,000 and below, that's a good outcome in terms of where we would hope to get to with this outbreak.

"But that's still horrible, it's still an enormous number of deaths and an enormous pressure on the service."

In comparison, Britain has about 8,000 deaths from seasonal flu every year.

Asked about when tough restrictions on social distancing would be lifted, he told the committee the evidence pointed to the virus returning.

"I can't tell you how big that's going to be," he said, assessing curbs would be in place for "months", without specifying an exact number.

Scientists from Imperial College London on Monday said social distancing measures could be in place for up to 18 months.