A new "yet more transmittable" strain of coronavirus has been detected in the UK.
Speaking at a Downing Street press conference, health secretary Matt Hancock said two cases have been discovered - both connected to people who have travelled from South Africa.
He said the new variant was "highly concerning".
Travel restrictions are being brought in immediately on travel from the country.
Mr Hancock said: "We have detected two cases of another new variant of coronavirus here in the UK. Both are contacts of cases who have travelled from South Africa over the past few weeks.
"We are incredibly grateful to the South African Government for the rigour of their science and the openness and the transparency with which they have rightly acted as we did when we discovered a new variant here.”
"This new variant is highly concerning because it is yet more transmittable.
"First we are quarantining cases and close contacts of cases found here in the UK.
"We are placing immediate restrictions on travel from South Africa.
"Anyone in the UK who has been in South Africa in the last fortnight must quarantine immediately."
Close contacts of people who have been in South Africa in the last fortnight are also being told to quarantine.
Mr Hancock added: "These measures are temporary while we investigate further this new strain which is shortly to be analysed at Porton Down.”
