Three cases have been recorded in England and three in Scotland but one of the England cases has not been identified.
A hunt is under way for one of the first people in the UK believed to have contracted a Brazilian coronavirus “variant of concern”.
The COVID-19 variant, first seen in the city of Manaus, is thought to spread more rapidly than the original virus and to be more capable of evading existing vaccines.
Three cases of the P.1 coronavirus variant have been confirmed in England and three in Scotland, Public Health England said.
Two of the cases in England come from a household in South Gloucestershire, where one person returned from Brazil in mid-February, before hotel quarantine measures were introduced.
The third case is unlinked and the whereabouts of the individual unknown, as they did not complete their test registration card, PHE said.
The person’s test was processed on 14 February so officials have said it is likely to have been taken a day or two before then.
It appears to have been done at home or as part of local surge testing, as regional test sites have staff checking contact details are provided.
Moves are also under way to contact the passengers on Swiss Air flight LX318 from Sao Paulo through Zurich to London Heathrow on 10 February.
And surge testing will be carried out in the Bradley Stoke, Patchway and Little Stoke areas of South Gloucestershire.
Scotland’s three variant cases have been identified in the Grampian region after three people flew from Brazil to Aberdeen via Paris and London in February.
All the other passengers who were on their final flight from London to Aberdeen have been contacted, the Scottish government said.
The Scotland and England cases are not thought to be connected.
There are concerns that the variant may be more resistant to existing vaccines.
Lawrence Young, a professor of molecular oncology, told Sky News of the variant: “We know it’s more transmissible. We also know it’s more resistant to the immune system – both vaccine induced protection and the natural protection that comes from previous infections is likely to be diminished by this variant, so it is a real concern.”
NHS England national medical director Professor Stephen Powis said vaccines can be quickly altered to tackle new strains.
“The new vaccines which are being used for COVID-19 can be adapted very rapidly so it’s likely that if we do need to change the vaccine that can be done in months, rather than years, which was the case with the more traditional vaccines,” he told the BBC.
It comes after the government said that 20 million people in the UK have had at least one dose of a coronavirus vaccine, described by Prime Minister Boris Johnson as a “huge national achievement”.
Meanwhile, a further 144 people had died within 28 days of testing positive for COVID-19 as of Sunday, bringing the official UK death toll to 122,849.