It is "absolutely vital" that people in areas of England where the South African variant of coronavirus has been identified minimise all social contact, the health secretary has said.
Speaking at the Downing Street news conference, Matt Hancock said the emergence of the variant was a "stark reminder the fight against this virus is not over yet" and that now was "no time to let things slip".
"There is currently no evidence to suggest this variant is any more severe," he said.
"But we need to come down hard on it and we will."
A total of 105 cases of the South African variant have been identified in the UK so far, with 11 of them not appearing to have links to international travel.
Household testing will be rolled out across parts of England due to concerns that the variant could be spreading in some regions.
The government said the "surge testing" scheme will begin in:
London - W7 (Ealing), N17 (Haringey) and CR4 (Croydon)
West Midlands - WS2 (Walsall)
East of England - EN10 (Broxbourne)
South East - ME15 (Maidstone) and GU21 (Guildford)
North West - PR9 (Preston)
Mr Hancock said it was "imperative" that people in these areas stay at home and take a test when it is offered to them, even if they have no symptoms.
"The most important thing is that people in the postcode areas outlined need to take extra special precautions," the health secretary said.
"It is absolutely vital that people in those areas minimise all social contact and get a test when the opportunity arises.
Mr Hancock added that "finding every single case" of the variant was the government's aim.