The decision means that from next week, any UK-bound arrivals will have to self-isolate for 10 days as soon as they enter the country, unless they can provide a negative test result after five days in quarantine.

Passengers must also be able to provide a negative Covid-19 test 72 hours before leaving their country of origin, and a completed Passenger Locator Form, which airline staff will request before take-off.

The measures will be enforced with additional checks when passengers land and “substantial fines” for those refusing to comply, Johnson said during a coronavirus press conference.

“It is precisely because we have the hope of that vaccine and the risk of new strains coming from overseas that we must take additional steps now to stop those strains from entering the country,” the PM said.

From 4:00am (GMT) on Friday the UK closed its borders to all travellers from South America and Portugal over fears of a new Covid-19 variant discovered in Brazil.

The variant, which has multiple mutations, was first detected in Japan in travellers from Brazil's Amazonas state, but it has not yet been found in the UK, a spokesperson for Public Health England (PHE) said on Friday.

However, eight cases of a different variant discovered in Brazil, with fewer mutations, have been detected in the UK, while there have been 35 confirmed cases of a Covid-19 strain from South Africa, according to PHE.

The government has taken action in response to new variants after a new Covid-19 strain was discovered in the UK in mid-December and spread rapidly across the country, replacing earlier forms of the virus.

The UK reported a further 55,761 cases of Covid-19 on Friday, as well as another 1,280 deaths within 28 days of a positive test, according to official data.