The UK has detected two cases of another new variant of 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.
This variant shares some similarities to the one that has already been detected in the UK, although they have evolved separately.
Both have a mutation - called N501Y - which is in a crucial part of the virus that it uses to infect the body's cells.
Prof Neil Ferguson, from Imperial College London, said: "I think the greatest concern of ours at the moment is the South African one.
At the Downing Street press briefing, Mr Hancock said the new variant was "highly concerning" and that anyone told to quarantine must avoid "all contact with any other person whatsoever".