It comes as the UK’s death toll from the coronavirus reached 55, with 1,543 people confirmed to have been infected.

Giving a press conference this evening, Mr Johnson said elderly people and those with the most serious health conditions should ‘shield’ themselves for 12 weeks.

‘It is now clear that the peak of the epidemic is coming, faster in some parts of the country than in others,’ he said ‘It looks as though London is a few weeks ahead.’

He said Londoners must ‘pay special attention to what we are saying about avoiding non-essential travel.’

The prime minister set out the need for ‘drastic action’ to tackle the ‘fast growth’ of coronavirus.

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He said that according to the Scientific Advisory Group for Emergencies (Sage) ‘it looks as though we are now approaching the fast growth part of the upward curve’ in the number of cases.

‘Without drastic action cases could double every five or six days,’ he said.

Mr Johnson said anyone who lives with someone who has a cough or a temperature should stay at home for 14 days.

He also urged unnecessary visits to care homes to stop to protect the vulnerable.

From tomorrow, mass gatherings will not be supported by emergency workers in the way that they are normally.

Mr Johnson added: ‘This advice about avoiding all social contact is
particularly important for people over 70, for pregnant women and for those with
some health conditions.’

The press conference comes as the World Health Organisation urged countries to ‘test, test, test’ every suspected case of Covid-19 as it is impossible to ‘fight a fire blindfolded’.

However, the UK’s policy is to test only those in hospital already, or those at a care home or prison infected with the virus.

England’s chief medical officer Professor Chris Whitty insisted that the UK’s testing regimen was robust.

He said there was ‘complete surveillance’ testing in intensive care, hospitals were also testing patients with pneumonia and GPs were testing in the community.

‘We do intend to continue to scale up testing,’ he said, adding efforts were already ‘substantial’ with more than 44,000 tests conducted.

At the moment tests were only useful for people who were currently sick, but it would be ‘transformational’ if there was a way to find out whether people had previously had it.

That would show what proportion of people can get the disease without any symptoms, he said, adding that Public Health England was ‘very rapidly’ developing such a test.

Professor Whitty added: ‘We really would encourage (people) not, if they have mild or moderate disease, to phone 111 because we need to protect the service for those who are in greatest need.

‘But, (it’s) really important to stress, if anyone’s health starts to deteriorate significantly then they should phone 111 or contact health services in the way that they usually would.’

He said: ‘We are enormously proud of what our colleagues in the NHS and Public Health have done so far to delay this in a safe way and an enormous amount of work has gone on to do that.

‘The next few weeks and months are going to be extraordinarily difficult for the NHS in all four nations.

‘We know that our colleagues will rise to this challenge, but we know it is going to be very hard indeed, but we have enormous faith in them.’