Everyone arriving in the UK will have to take PCR tests after two cases of the Omicron variant were detected in England.
And more countries have been added to the red list in a bid to slow the spread of the virus.
Two cases of the Omicron variant feared to have higher re-infection rates have been detected in the UK, as the Government added four more southern African nations to the travel red list.
Prime Minister Boris Johnson said anyone arriving in the UK will be asked to take a PCR test for Covid-19 on the second day and must self isolate until they provide a negative test.
At the same time, the rules on face coverings in shops and on public transport will be tightened, the Prime Minister said.
Prime Minister Boris Johnson announced the move at a press conference on Saturday.
"This is the responsible course of action to slow down the seeding and the spread of this variant," he said.
Mr Johnson added that the measures would be reviewed in three weeks, when scientists should know more about the new strain.
"We need to protect gains we have made so far," he said.
Health officials revealed earlier on Saturday that two cases of the feared Omicron variant had been found in the UK - one in Brentwood, Essex and another in Nottingham.
The UK Health Security Agency confirmed the cases, which are both believed to be connected and linked to travel to southern Africa, after genomic sequencing overnight.
The government has now added four more southern African nations to the travel red list: Malawi, Mozambique, Zambia and Angola.
They join South Africa, Botswana, Eswatini, Lesotho, Namibia and Zimbabwe.
Anyone returning from those countries will have to quarantine in a hotel for 10 days.
The Prime Minister told a Downing Street press conference on Saturday: â€œWeâ€™re not going to stop people travelling, I want to stress that, weâ€™re not going to stop people travelling, but we will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result.
â€œSecond, we need to slow down the spread of this variant here in the UK, because measures at the border can only ever minimise and delay the arrival of a new variant rather than stop it all together.
â€œWe will require all contacts of those who test positive with a suspected case of Omicron to self-isolate for 10 days regardless of your vaccination status.
â€œWe will also go further in asking all of you to help contain the spread of this variant by tightening up the rules on face coverings in shops and on public transport.â€