The UK has detected two cases of another new variant of coronavirus, the health secretary Matt Hancock says.
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".
He said the country "cannot go through what we went through in the early days of the Aids pandemic".
Scientists in South Africa say the variant has "spread rapidly" and became the dominant form of the virus in parts of the country.
The variant is still being analysed, but the data are consistent with it spreading more quickly.
It was detected for the first time in the UK on Tuesday.