Boris Johnson has shut all travel corridors to the UK from 4am on Monday, meaning no one can enter the country without a negative Covid test result and everyone must quarantine on arrival.
The test must have been taken within the previous 72 hours.
The prime minister tightened the border amid concerns that a new Brazilian variant of coronavirus could be imported to the UK.
Once people arrive in the country they will also have to begin 10 days of isolation, unless they again test negative for Covid after five days, under the “test for release” programme.
The government is also planning more spot checks to ensure people are remaining in isolation.
The rules also apply to British nationals, meaning any hoping to return from Monday morning will not be able to if they test positive for Covid, unless they meet limited criteria for exemptions.
Many countries around the world were on the travel corridor list, meaning they could enter the UK without having to quarantine or have a test, including Australia, Hong Kong, Iceland, Ireland, Japan, Saudi Arabia, South Korea and Thailand.
It comes after Johnson was sharply criticised by Commons home affairs committee chair Yvette Cooper over protection from new Covid variants at the border.
The new regime will be in place until at least February 15 while work to understand the threat posed by mutations of the virus continues.
It comes after all flights from Brazil and South Africa were banned in response to new variants emerging in those countries.
Johnson told a Downing Street press conference on Friday: “It’s 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.
“Yesterday we announced that we’re banning flights from South America and Portugal and to protect us against the risk from as-yet-unidentified strains we will also temporarily close all travel corridors from 4am on Monday.
“Following conversations with the devolved administrations we will act together so this applies across the whole of the UK.”
One Brazilian variant has already been detected in the UK but it is not the mutation that sparked the South America travel ban.