Boris Johnson has urged the British public to take unprecedented peacetime measures, including avoiding all unnecessary contact and travel and staying away from pubs and theatres, following expert modelling which suggests the approach could cut the estimated coronavirus death toll from 260,000 to 20,000.

The prime minister said the disease was now “approaching the fast growth part of the upward curve,” with London in the vanguard. Deaths in the UK rose from 35 to 55 on Monday, with half of diagnosed cases in the capital.

He announced a series of stringent steps, including 14 days of self-isolation for a whole household if anyone displays symptoms of the disease. Anyone who can work from home, should, he said.

After several days in which the government faced growing questions over why it had eschewed the drastic measures taken in other affected countries, such as school closures, Johnson used the first of his new daily press conferences to announce the stringent new plan.

“Now is the time for everyone to stop non-essential contact with others and to stop all unnecessary travel. We need people to start working from home where they possibly can. And you should avoid pubs, clubs, theatres and other such social venues,” he said.

The prime minister said the advice to avoid unnecessary social contact was “particularly important for people over 70, for pregnant women and for those with some health conditions” – and further information would be published for the most vulnerable, including those with serious underlying health conditions, in the coming days. The government said this would ensure they were “shielded” from the virus for a period of 12 weeks.

The chief scientific adviser said the course of Covid-19 in the UK was about three weeks behind Italy, which has seen more than 1,800 deaths.

On a day of rapid developments in the pandemic:

• The World Health Organization’s director general, Tedros Adhanom Ghebreyesus, urged governments to continue tracking cases of the virus, saying, “test, test, test”. The UK promised to scale up testing but continues with a policy not to test suspected sufferers, including frontline health workers, unless they are being treated in hospital.

• Donald Trump banned gatherings of more than 10 people while G7 leaders announced that they would do “whatever is necessary” to combat the virus.

• Johnson acknowledged that more action would probably be needed to protect low-paid workers, saying “nobody should be penalised for doing the right thing”.

Jeremy Corbyn was due to meet the prime minister on Monday evening. A Labour spokesperson said he would call for measures to aid the lowest-paid, and “ask the government to recognise this as the critical moment to invest in public services and to introduce urgent financial support for those affected by the coronavirus”.

If the government had stuck to the plans announced last week, Covid-19 could have cost 260,000 lives, according to teams of modellers from Imperial College who have run computer predictions of the impact of different interventions. The government’s strategy is based on their work and similar modelling from the London School of Hygiene and Tropical Medicine.

Prof Neil Ferguson from Imperial College and his colleagues call that scenario 1, the mitigation strategy. It was not until they received new and sobering data from Italy, where the health service is struggling to save lives, that they understood the potential death toll and the horrendous impact on the NHS of that strategy which allowed people to mix as usual. In Italy, 30% of hospitalised patients with the virus have needed intensive care. The NHS would not have been able to cope.

Instead of letting social contact at pubs, clubs, restaurants and the theatre continue, and allowing people travel and go to work, they modelled scenario 2, which they call suppression. Under that strategy, the number of deaths would be driven down to 20,000 or lower – possibly even a few thousand. But the social restrictions the government has now announced will have to stay in place for at least five months and maybe longer.

“It does really leave the world in a quandary,” said Ferguson, whose work is also being studied by France and the US. “We expect, when we lift these measures, for transmission to resume.” He said this would buy a period of time with very low transmission but it meant very little build-up of herd immunity, which was thought to interest the government last week.

Pubs, clubs and theatres will not be ordered to close, but will be expected to do so voluntarily.

But the advice to stay away risks triggering thousands of closures and hundreds of thousands of job losses unless the government provides urgent financial support, industry figures said.

The government is poised to announce fresh help to support business later this week amid signs that Covid-19 is inflicting what Johnson said was a “severe blow” on the economy.

Sources said Rishi Sunak, the chancellor, was in discussions with cabinet colleagues and that, following the £12bn package of support provided in the budget, he would appear at Tuesday’s press conference with the prime minister to announce more measures to help businesses.

The new governor of the Bank of England, Andrew Bailey, gave a strong hint of additional stimulus following last week’s cut in interest rates to a joint record low of 0.25%.

Flanked by Sir Patrick Vallance, the government’s chief scientific adviser, and Chris Whitty, the chief medical officer, Johnson said: “Many people, including millions of active people over 70, may feel, listening to what I’ve just said, that there is something excessive about these measures. But I have to say I believe they are overwhelmingly worth it, to slow the spread of the disease, reduce the peak, to save lives, minimise suffering and to give our NHS the chance to cope.”

Quick guide What to do if you have coronavirus symptoms in the UK Show Hide Symptoms are defined by the NHS as either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you've started coughing repeatedly NHS advice is that anyone with symptoms should stay at home for at least 7 days.

If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home. After 14 days, anyone you live with who does not have symptoms can return to their normal routine. But, if anyone in your home gets symptoms, they should stay at home for 7 days from the day their symptoms start. Even if it means they're at home for longer than 14 days. If you live with someone who is 70 or over, has a long-term condition, is pregnant or has a weakened immune system, try to find somewhere else for them to stay for 14 days. If you have to stay at home together, try to keep away from each other as much as possible. After 7 days, if you no longer have a high temperature you can return to your normal routine. If you still have a high temperature, stay at home until your temperature returns to normal. If you still have a cough after 7 days, but your temperature is normal, you do not need to continue staying at home. A cough can last for several weeks after the infection has gone.

Staying at home means you should: not go to work, school or public areas

not use public transport or taxis

not have visitors, such as friends and family, in your home

not go out to buy food or collect medicine – order them by phone or online, or ask someone else to drop them off at your home You can use your garden, if you have one. You can also leave the house to exercise – but stay at least 2 metres away from other people. If you have symptoms of coronavirus, use the NHS 111 coronavirus service to find out what to do. Source: NHS England on 23 March 2020

The measures, which Johnson described as “draconian,” were largely welcomed by medical experts.

Venki Ramakrishnan, president of the Royal Society, said: “The government’s announcement of further increased restrictions to slow the spread of coronavirus is very welcome. Compliance with the new guidelines, and those previously announced, is essential to slow the rate of infection so as not to overburden health services beyond capacity.”

Whitty said the country had to be prepared for tough restrictions to be in place for “the long haul”. He said: “People should be thinking of a minimum of weeks to months and, depending how it goes, it may be longer. It’s really important people realise they are in for the long haul on this.

“But this is really important, if we are to defend the ability of the NHS to treat people, if we are actually to minimise mortality, we have got to see this as a long game.” The trio also made clear that school closures might have to be considered in the weeks ahead.

On people isolating for 14 days, Johnson said: “That means that if possible you should not go out, even to buy food or essentials, other than for exercise and in that case at a safe distance from others.” Whitty said this was vital to help limit the spread, and relieve pressure on the NHS.

Whitty defended the UK’s testing regime, following criticism it has been too slow and minimal. At a press conference earlier on Monday, Tedros, the head of the WHO said: “You cannot fight the fire blindfolded. And we cannot stop this pandemic, if we don’t know who is infected. We have a simple message for all countries – test, test, test.”

Whitty said: “We do intend to continue to scale up testing,” adding efforts were already “substantial”, with more than 44,000 tests conducted.