The UK has identified 77 cases of the coronavirus variant first detected in South Africa, the health secretary has said.
Cases are linked to travellers arriving in the UK, rather than community transmission, Matt Hancock added.
He told the BBC’s Andrew Marr cases were under “very close” observation and enhanced contact tracing was under way.
Meanwhile, Mr Hancock said 75% of over-80s and three quarters of care homes in the UK have received a first Covid jab.
Both the Pfizer-BioNTech and Oxford-AstraZeneca vaccines require two doses, and figures so far reflect those given the first dose.
All viruses, including the one that causes Covid-19, mutate, and variants have been first located in the UK, South Africa and Brazil.
The South Africa variant has been found in at least 20 other countries, including the UK.
Mr Hancock said: “At the moment it (cases) is all linked to travel.” He added: “That’s why we have got such stringent border measures in place against movement from South Africa.”
The UK closed all travel corridors last week until at least 15 February, with almost all travellers arriving in the country now required to show proof of a negative Covid-19 test to be allowed entry.
Prime Minister Boris Johnson has not ruled out bringing in tougher measures at UK borders, telling a Downing Street news conference on Friday: “We don’t want to put that (efforts to control Covid) at risk by having a new variant come back in.”
Ministers will discuss on Monday whether to tighten border restrictions further, including the possibility of hotel quarantines for travellers.
Mr Hancock said: “We have got to be cautious at the borders.”
Government data on 14 January showed there were 35 confirmed cases of the South Africa variant identified in the UK, and a further 12 “probable” cases.
Mr Hancock said nine cases of the Brazil variant had been found in the UK, adding “we are monitoring each and every one very closely”.
Shadow foreign secretary Lisa Nandy told the BBC’s Andrew Marr Show that Labour had been “pushing the government to take tougher measures at the border since last spring”.
She said: “We would fully expect the government to bring in tougher quarantine measures, we would expect them to roll out a proper testing strategy and we would expect them as well to start checking up on the people who are quarantining.
“Only three out of every hundred people who are asked to quarantine when they arrive into the UK actually face any checks at all – that’s just simply not sufficient.”