British nationals should avoid all non-essential foreign travel to tackle the spread of coronavirus, the Foreign and Commonwealth Office (FCO) has advised.
The restrictions will be in place for 30 days initially but could be extended, the foreign secretary said.
Meanwhile the Euro 2020 tournament has been postponed and all Church of England services have been suspended.
Chancellor Rishi Sunak is expected to unveil financial measures to ease the burden of the virus on UK businesses.
In another day of fast-changing developments across the globe:
- China has reacted angrily to a tweet from US President Donald Trump that described the coronavirus as "Chinese"
- French residents face a fine if they are outside without good reason
- The number of deaths in Italy has passed 2,000 and the total in Spain has reached nearly 500
- The International Olympic Committee is meeting later to discuss the summer games in Tokyo
- All horse racing in Great Britain will be suspended from Wednesday until the end of April
- Follow live updates here
It is the first time the FCO has advised against foreign travel anywhere in the world.
"UK travellers abroad now face widespread international border restrictions and lockdowns in various countries. The speed and range of those measures across other countries is unprecedented," Foreign Secretary Dominic Raab said in a statement.
The Archbishops of Canterbury and York have said all Church of England services and other public worship should be suspended until further notice - although weddings and funerals can still go ahead, a spokesman said.
The number of people who have died with the virus in the UK has reached 56, after a second death was confirmed in Scotland.
Some 1,950 people have tested positive for the virus in the UK, according to the latest Department of Health figures - but the actual number of cases could be as high as 55,000.
The government's chief scientific adviser Sir Patrick Vallance told the health select committee that a death rate of one fatality for every 1,000 cases was a "reasonable ballpark" figure, based on scientific modelling.
More than 50,000 people have been tested for the virus in the UK, but the government is primarily testing people who are in hospital. This means many people who have mild symptoms may never be diagnosed with the virus.
British people currently abroad do not have to immediately return to the UK - except for those in a few countries detailed in the FCO's travel advice.
But the FCO said travellers should bear in mind that flights could be cancelled at short notice as foreign countries grapple with restrictions being imposed by their own authorities.
Mr Raab said anyone who is still considering foreign travel should be "realistic about the level of disruption they are willing and able to endure".
The foreign secretary said the government would issue advice on how the flow of food and goods to the UK can be maintained.
Mr Raab said staff working on shipping routes should continue to do so as their travel was "essential".
The travel advice for British nationals has in part been brought in because of the stringent social distancing measures announced by Boris Johnson on Monday.
The key new measures the prime minister announced included:
- Everyone should avoid gatherings with friends and family, as well as large gatherings and crowded places, such as pubs, clubs and theatres
- People should avoid non-essential travel and work from home if they can
- All "unnecessary" visits to friends and relatives in care homes should cease
- People should only use the NHS "where we really need to" - and can reduce the burden on workers by getting advice on the NHS website where possible
- By next weekend, those with the most serious health conditions must be "largely shielded from social contact for around 12 weeks"
- If one person in any household has a persistent cough or fever, everyone living there must stay at home for 14 days
- Those people should, if possible, avoid leaving the house "even to buy food or essentials" - but they may leave the house "for exercise and, in that case, at a safe distance from others"
While schools will not be closed for the moment, a union leader has described the "intolerable pressure" teachers are under as a result of the lack of clarity about pupil and staff safety.
Businesses have also called for more details about the measures. Many small firms are unlikely to have insurance cover to compensate them for loss due to the virus, experts have warned.
Chancellor Rishi Sunak is expected to appear at the now daily Downing Street news conference later, where he will unveil more financial plans to help the economy during the pandemic.
'A war we must win'
At a cabinet meeting earlier, Mr Johnson told ministers: "We are engaged in a war against the disease which we have to win."
Downing Street said the prime minister urged the government to support businesses "through what will be hugely challenging times".
Mr Johnson has set up a daily meeting about the virus, which causes the Covid-19 disease, which he will chair.
In other developments in relation to coronavirus:
- The Queen will move to Windsor Castle on Thursday - a week earlier than usual at this time of year. She is likely to stay there beyond the Easter period, Buckingham Palace said
- Ofsted inspections are being halted to "remove unnecessary burdens" on schools, the Education Secretary Gavin Williamson said
- The Royal Albert Hall is the latest cultural venue to close its doors - following theatres, museums, and most cinemas
- Workers at Amazon's UK warehouses are being told to do overtime to tackle huge demand
- And the UK's mobile networks are experiencing problems as people follow government guidance to work from home
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