Britons have been told to stop “all unnecessary travel” and stay away from pubs and clubs, as the number of coronavirus victims escalates.

And up to 1.5 million people with the “most serious health conditions” must avoid almost all social contact for 12 weeks, from this weekend.

In London – a virus hotspot – the crackdown has been underlined, with everyone urged to “pay special attention“ to the call to reduce social contacts.

Addressing the nation as the UK death toll rose to 55, Boris Johnson said: “Even if you don’t have symptoms, and no-one in your households has symptoms, there is more we need you to do now.

“Now is the time for everyone to stop non-essential contact with others, and to stop all unnecessary travel.

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“We need people to start working from home when they can. You should avoid pubs, clubs, theatres and other such venues.”

Mr Johnson said that the advice to avoid unnecessary contact was “particularly important” for the over-70s, pregnant women and those with certain health conditions.

The new restrictions also include entire households being told to stay in isolation for 14 days if anyone displays the symptoms of persistent cough or fever. If a person who displays the symptoms lives alone, they should self-isolate for seven days.

“We’re entering a fast growth period or we’re on the cusp of one,” said Patrick Vallance, the chief scientific adviser.

Quizzed on how the government would enforce the measures, the prime minister suggested venue owners would take the “logical step” of closing their premises.

“When it comes to enforcement we have the powers if necessary, but I don’t believe it will be necessary to use those powers,” he said.

However, he did suggest curfews might be introduced as the outbreak gathers pace, saying: “We are keeping all measures under review.”

And he said that from Tuesday this week, emergency workers and ambulances will no longer be deployed at mass gatherings such as sports events.

Chris Whitty, the chief medical officer, warned the measures would have to last for a long period, saying: “This is going to go on for some time. We shouldn’t be under any illusion that if we just do this for a couple of weeks, it’s going to be sufficient. This will have to be a prolonged period.”

He argued that school closures were not necessary at present, but could be implemented “at the right time, in the right way, at the right stage” of the outbreak.

He warned there would be direct deaths from the virus, but also indirect deaths – as NHS intensive care units were overwhelmed – and from negative health impacts, as people stayed indoors and became isolated.

Professor Whitty also defended the government’s record on testing for coronavirus, after the World Health Organisation called for it to be dramatically stepped up, urging world leaders to “test, test, test”.

The prime minister addresses the nation (Getty)

Following criticism that the UK is only testing people who are so sick they need medical treatment – rather than anyone who is isolating themselves – Mr Whitty said that the process would “scale up every single week from this day in”.

So far, more than 44,000 people have been tested, with 1,543 positive and more than 42,000 negative. The number of deaths across Britain rose by 20 since Sunday, including the first death in Wales.

Sir Patrick also conceded the experts could not be certain that a person would have immunity after recovering from the virus, after being alerted to a case in Japan of someone contracting it twice.

“In any infectious disease, there are cases where you can catch it again ... there will be occasions very rare where somebody will get it more than once,” he warned.

Mr Johnson said: “What we are announcing today is a very substantial change to the way we want people to live their lives. I can’t remember anything like it in my lifetime. I don’t think there’s been anything like it in peacetime.

“It is a very considerable psychological change we are asking the public to make, but I have absolutely no doubt we can do it, we can do it together.”

The prime minister said the government would do “everything we can” to ensure that businesses have the support they need to survive the epidemic.

Following talks with fellow leaders of G7 nations, he said there was widespread agreement on the need to ensure business access to liquidity.

“Everybody sees that this is potentially going to be a severe blow to the economy, everyone can see the risk this poses to demand and to supply,” he said.

“But if we get it right and we work together, we can ensure that it is a short-term problem. The fundamentals of the UK economy are very solid. This is unlike 2008 – there isn’t a systemic problem in the economy, if we can get the disease under control in the way we are describing, there is absolutely no reason why economies worldwide shouldn’t come roaring back.”

In a joint statement released by Washington, the G7 leaders said they had agreed to “marshall the full power of our governments” to deliver a co-ordinated health response to protect people from Covid-19, to restore confidence and economic growth and to protect jobs.

“By acting together, we will work to resolve the health and economic risks caused by the Covid-19 pandemic and set the stage for a strong recovery of strong, sustainable economic growth and prosperity,” said the statement, agreed by Mr Johnson, US president Donald Trump and leaders of France, Germany, Italy, Canada and Japan.

Mr Johnson said there were no immediate plans to shut down parliament, but he revealed that talks are under way on possible changes to the way Westminster functions to reduce the likelihood of spreading the virus.

And he indicated that MPs and peers aged over 70 – who include Labour leader Jeremy Corbyn – should join other elderly people in observing the advice on social distancing particularly rigorously.

“We will want to make sure that democracy carries on and we protect our parliamentary institutions,” he said. “But clearly those who are vulnerable should receive the protections they need.