Boris Johnson has confirmed travellers touching down in the UK will require a PCR test on arrival.
The move comes as authorities seek to contain the spread of a new Covid-19 variant, known as Omicron.
Two cases of the variant have already been identified in Essex and Nottingham.
Targeted testing and contact tracing is now underway, the government said.
Early indications suggest this variant may be more transmissible than the Delta variant and current vaccines may be less effective against it.
A rapid rise in infections in South Africa has been attributed to the spread of this new variant of Covid-19.
The prime minister confirmed a PCR test will be necessary for all travellers by the end of the second day after arriving in the country.
Passengers will also be asked to self-isolate until they have secured a negative result.
Johnson said, as this stage, people are not going to be prevented from travelling to, or returning from, overseas.
He added the restrictions on travel “sound tough,” but insisted the new measures were “targeted and appropriate”.
The move has been described as a setback to the travel sector.
An ABTA spokesperson said: “While ABTA understands that this is a rapidly evolving situation and public health must come first, the decision to require all arrivals to take a PCR test and self-isolate until a negative result is returned is a huge blow for travel businesses, many of whom were only just starting to get back on their feet after 20 months of severe restrictions.
“These changes will add cost to people’s holidays which will undoubtedly impact consumer demand and hold back the industry’s recovery, so it’s vital that this decision is kept under careful review and restrictions are lifted promptly if it becomes clear there is not a risk to the UK vaccination programme.”
Rory Boland, Which? Travel editor, warned there is still room for improvement in the PCR test market.
He added: “As Which? research has consistently shown, the private testing market isn’t fit for purpose – with serious implications not just for travellers but public health more generally.
“Testing firms have struggled to provide tests on time over the past year, so it is hard to have confidence they will be able to cope with this spike in demand at short notice.
“Now that the government has taken the decision to make PCR tests mandatory, it must take steps to properly regulate the marketplace and implement the Competition & Markets Authority’s (CMA) recommendations so that consumers can have confidence they are booking with a provider they can rely on.”