Health Secretary Matt Hancock says the UK has detected two cases of another new variant of the coronavirus.
The cases in London and north west England are contacts of people who travelled to South Africa, where the variant was discovered.
Travel restrictions with South Africa have been imposed.
Anyone who has travelled there in the past fortnight, and anyone they have been in contact with, are being told to quarantine immediately.
The variant has been causing mounting concern in South Africa, where health minister Zweli Mkhize warned that “young, previously healthy people are now becoming very sick”.