People entering England and Scotland will have to show a negative COVID-19 test result starting next week as authorities try to ramp up protection against the more infectious strain of the coronavirus.
Passengers arriving by boat, plane or train will have to take a test a maximum of 72 hours before departure, mirroring measures taken last year by other countries around the world.
"We already have significant measures in place to prevent imported cases of COVID-19, but with new strains of the virus developing internationally we must take further precautions," Transport Secretary Grant Shapps said.
He added that there were concerns that vaccines might not work properly against the highly transmissible variant of the coronavirus discovered in South Africa, echoing recent comments from other government officials.
On Thursday, Britain said it would extend a ban on travellers entering England from South Africa to include other southern African countries, and non-essential travel in and out of the United Kingdom is restricted.
Prime Minister Boris Johnson ordered a new lockdown for England this week after a surge in cases linked to another variant of the virus believed to have originated in the country.
Scotland, which like the rest of the United Kingdom has tight COVID-19 restrictions in place, said it too would require travellers to show negative tests and the rule is also expected to be applied by Wales and Northern Ireland.
Passengers from many countries are currently required to self-isolate for 10 days, or five if they pay for a private test and test negative. Those requirements will remain in place after the new pre-departure testing rule comes into effect.
Britain's airlines industry recognised the need to introduce pre-departure testing but said it should be only a short-term, emergency measure.
"Once the roll-out of the vaccine accelerates, the focus must be on returning travel to normal as quickly as possible in order to support the UK's economic recovery," said Tim Alderslade, chief executive of Airlines UK, an industry group.
"What we'd like to see is that testing before you take off becomes the standard as an alternative to quarantine," said John Holland-Kaye, chief executive of Britain's biggest airport Heathrow.
Travel to and from Britain has been almost wiped out by COVID-19 and the quarantine requirements, leaving many airlines and airports fighting for survival.
Exemptions to the new testing requirement rule in England would be offered to hauliers, children under 11, crews and people travelling from countries where tests are not available.
Passengers will be subject to a fine of 500 pounds ($678.30) if they fail to comply with the new regulations.