UK officials believe up to 10,000 people are already infected with the coronavirus in Britain - 20 times higher than the officially confirmed number.

The number of confirmed cases is 590, but many people carrying the virus are likely not to have been tested.

'It's much more likely that we've got somewhere between 5,000 and 10,000 people infected at the moment,' the UK's chief scientific advisor Patrick Vallance told a press conference on Thursday.

Boris Johnson warned: 'Many more families are going to lose loved ones before their time.'

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The UK government believes that up to 10,000 have already been infected with the coronavirus, despite there being just 590 confirmed cases in the country.

The government's chief scientific advisor, Patrick Vallance, said on Thursday that the number of actual cases was likely to be much higher than the number of confirmed cases because many people have not yet been tested.

Speaking alongside Boris Johnson, the prime minister, Vallance said the fact that more than 20 patients were in intensive care meant that it was statistically likely that thousands of cases were undiagnosed.

"There are more than 20 patients on intensive care units," he said. "If you calculate what that really means in terms of the total number, it's much more likely that we've got somewhere between 5,000 and 10,000 people infected at the moment."

In the same press conference, the prime minister warned that the coronavirus outbreak was "this is the worst public health crisis for a generation."

"We've done what can be done to contain this disease and this has brought us valuable time. But it's now a global pandemic and the number of cases will rise sharply.

"Indeed the true number of cases is higher — perhaps much higher — than the number of cases we have confirmed with tests.

"I've got to be clear, this is the worst public health crisis for a generation. Some people compare it to seasonal flu. Alas that is not right.

"Owing to the lack of immunity, this disease is more dangerous and it's going to go further. Many more families are going to lose loved ones before their time."

The UK will now move to the "delay" phase of its coronavirus action plan, Johnson confirmed, meaning the government will ask people showing symptoms associated with coronavirus to stay at home for seven days.

It will also consider other measures, such as banning mass gatherings.