Travellers from across South America have been banned from entering the UK amid growing concerns about a mutant coronavirus strain which has emerged in Brazil.
The ban which, also covers the Central American state of Panama and Portugal – due to its strong travel links with Brazil – and the former Portuguese colony of Cape Verde, came into force at 4am on Friday.
Scientists analysing the Brazilian variant believe the mutations it shares with the new South African strain seem to be associated with a rapid increase in cases in locations where there have already been large outbreaks of the disease.
Transport Secretary Grant Shapps described the ban as a "precautionary" move to ensure the vaccination programme rolling out across the UK was not disrupted by new variants of the virus.
"We don't want to trip up at this late stage. We don't have cases at the moment but this is a precautionary approach," he told BBC News.
"We want to make sure that we do everything possible so that vaccine rollout can continue and make sure that it is not disturbed by other variants of this virus."
British and Irish nationals and others with residence rights are exempted from the measure, which was backed by the Scottish Government, though they must self-isolate for 10 days along with their households on their return.
There is an exemption also for hauliers travelling from Portugal to allow the transport of essential goods.
Shadow home secretary Nick Thomas-Symonds said the ban was a "necessary step" but accused ministers of incompetence and "lurching from one crisis and rushed announcement to another".
The move came as the latest figures showed the number of people across the UK to have received the first dose of a Covid-19 vaccine has passed 2.9 million.
Public Health England (PHE) also released data showing infection rates had fallen in most regions of England across all age groups apart from the over 80s, in a further sign that lockdown measures are having an impact.
At the same time, however, the PHE surveillance report noted that there were more people being admitted to hospitals and intensive care units.
NHS England said around one in five major hospital trusts in England had no spare adult critical care beds on January 10.
The time lag between a fall in cases and an impact on the death toll means grim figures are likely to remain a factor for some time.
The latest official figures showed a further 1,248 people had died within 28 days of testing positive for Covid-19 in the UK as of Thursday.
Meanwhile, Boris Johnson is coming under renewed pressure from Tory lockdown sceptics to set out how he will ease the restrictions on people's liberties as cases come down.
Former minister Steve Baker – who co-ordinated opposition to Theresa May's Brexit deal – suggested Mr Johnson's leadership would be "on the table" if there was not a change of direction.
He urged colleagues on the Covid Recovery Group to make their views known to the chief whip, saying: "People are telling me they are losing faith in our Conservative Party leadership because they are not standing up for our values as a party.
"If we continue forward with a strategy that hammers freedom, hammers the private sector, hammers small business owners and hammers the poor, inevitably the Prime Minister's leadership will be on the table: we strongly do not want that after all we have been through as a country."
After his comments leaked, Mr Baker later tweeted: "I am clear Boris is the only person to lead us out of these difficulties and I support him in that endeavour."