The UK had "no time to lose" in changing tactics in order to prevent thousands of deaths and the NHS being overwhelmed, scientists have said.
The Imperial College Covid-19 response team – which is one of several scientific teams advising ministers – published a paper showing that 250,000 people could die if efforts were focused only on delaying and slowing down the spread of Covid-19.
The paper analysed the most up-to-date data from Italy and the UK and concluded that the only "viable strategy" was a Chinese-style policy of "suppression" of the virus, elements of which have now been adopted in the UK.
It said: "In the UK, this conclusion has only been reached in the last few days, with the refinement of estimates of likely ICU (intensive care unit) demand due to Covid-19 based on experience in Italy and the UK (previous planning estimates assumed half the demand now estimated) and with the NHS providing increasing certainty around the limits of hospital surge capacity.
"We therefore conclude that epidemic suppression is the only viable strategy at the current time."
However, the team warned that even suppression of the virus has risks, as "we predict that transmission will quickly rebound if interventions are relaxed" and some tactics may need to therefore stay in place until a vaccine is available, which could be 12 to 18 months away.
One of the lead authors of the study, Professor Neil Ferguson, told BBC Radio 4's Today programme it had become apparent that the previous tactics being adopted – which aimed to slow down the spread – would still result in a "very large number of deaths and the health system being overwhelmed".
He said the science was continually shifting as more data became available, adding: "Initially when we came up with these sort of estimates they were viewed as what's called a reasonable worst case, but as information has been gathered in recent weeks from, particularly Italy, but other countries, it's become increasingly clear it's not the reasonable worst case, it's the most likely scenario."
Prof Ferguson said calculations from NHS planners on how much they could increase capacity in critical care played a key role in the research.
"Whilst they are planning a major expansion of that – cancelling elective surgery, building new beds, getting new ventilators – it just isn't enough to fill the gaps that would be left," he said.
"We are left with no option but to adopt this more draconian strategy."
Prof Ferguson said the Government had got the timing about right but warned there was no time to lose.
Asked if more draconian measures should have come earlier, he told Today: "I think we are still behind the epidemic seen in other European countries, so there's always a balancing act involved in these sort of measures in order to balance the impact of those measures, the costs on the economy against the impact on the epidemic.
"I overall think we have got the timing about right.
"I think we're about three weeks or so behind Italy, two weeks behind France and Spain, so we are making these decisions in a more timely manner than other European countries but certainly there wasn't any time to lose."
The stark warning came after Boris Johnson on Monday unveiled unprecedented peacetime measures to try to control the spread of Covid-19.
They were announced as the death toll of people with coronavirus in the UK reached 55.
In the first of his daily No 10 press conferences, the Prime Minister called on people to stay away from pubs, clubs and theatres and to avoid all non-essential contacts and travel, while those who live with somebody who is ill should stay home for 14 days.
In other developments:
– Chancellor Rishi Sunak will set out a new package of support for businesses hit by the outbreak less than a week after announcing £12 billion of emergency funding in the Budget
– The Government will lay out emergency legislation expected to include powers for police to detain people to stop them spreading the virus and allowing hospitals to send patients home to free up beds
– All non-essential access to the Houses of Parliament has been stopped and MPs and peers over the age of 70 or with underlying health problems have been urged to heed Government advice
– The Prime Minister held a conference call with manufacturing firms and organisations urging them to step up production of ventilators and other vital medical equipment
– The National Association of Funeral Directors is meeting the Cabinet Office on Tuesday to discuss how its members "can cope with extra number of deaths resulting from people being infected with Covid-19".
Special guidance will be issued by the NHS for the 1.4 million people most at risk from the disease – including the elderly and those with underlying health conditions – on further measures they need to take to "shield" themselves.
A World Health Organisation (WHO) special envoy on the coronavirus welcomed the Government's decision to introduce tougher measures.
Dr David Nabarro told the Today programme: "The thinking about the possibility of further more severe outbreaks coming later was perfectly valid, however as it became clear how quickly the virus has been advancing in other European countries a shift in position was absolutely right.
"I'm really pleased this has happened.
"We are just dealing with something that's so new with so many things we don't know that we have to be prepared for a change in tack from time to time even though it's distressing."
New #coronavirus advice:
▶️ We advise against people using public transport, particularly during rush hour. Instead people should vary the time they travel.
We strongly advise this for people aged 70 and over, pregnant women, or anyone with underlying health conditions. pic.twitter.com/YrYOc9guIx
— Department of Health and Social Care (@DHSCgovuk) March 16, 2020
In response to the Imperial paper, a Government spokesman said the recommendations put forward by its Scientific Advisory Group for Emergencies (Sage) were in line with "best current evidence".
"This is a very fast-moving situation. In order to give the most robust scientific advice Sage draws upon and considers a range of evidence and views to reach its recommendations," the spokesman said.
At his news conference, Mr Johnson said the latest Government measures represented a "very substantial change" in the way it was asking people to live their lives that was unprecedented in peacetime.
He said they represented "a very considerable psychological, behavioural change" but he said he had "absolutely no doubt" the country could do it.