Health officials are trying to trace one person in England who has been infected with a concerning variant of coronavirus first found in Brazil.
They are one of six cases of the P1 variant found in the UK in February.
The person did not complete a test registration form so remains unidentified – prompting an appeal for anyone without a result from a test on 12 or 13 February to come forward.
Labour said there had been a lack of a “comprehensive” border system.
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The P1 variant was first detected in travellers to Japan from Manaus in northern Brazil in January.
Health Secretary Matt Hancock will hold a meeting later to update MPs from all parties about the variant.
There have been concerns vaccines may not be as effective against the variant, but NHS England’s Prof Stephen Powis said vaccines could be “rapidly adapted”.
Dr Susan Hopkins, from Public Health England (PHE), said the UK was more advanced than many other countries at identifying the variants and mutations, and therefore was able to act quickly.
In an attempt to find the unidentified person in England who was infected with the P1 variant, officials are asking anyone who took a test on 12 or 13 February and who has not received a result or has an uncompleted test registration card to come forward immediately by calling 119.
Vaccines Minister Nadhim Zahawi said PHE was working with the postal service to try to locate the unidentified person.
“We need to find them as soon as possible,” he told BBC Breakfast.
Mr Zahawi defended the testing system as “pretty robust” and said the UK’s strategy of using surge testing to help identify cases of the variant was similar to other countries such as Australia.
The unidentified England case is not linked to five other UK cases.
Three of those cases are Scottish residents who flew to Aberdeen from Brazil via Paris and London, the Scottish government said.
Holyrood said the three people tested positive while self-isolating. Other passengers who were on the same flight to Aberdeen are now being contacted.
The other two cases in England are from the same household in South Gloucestershire after someone returned from Brazil on 10 February – five days before the government’s hotel quarantine rule came into force.
Two other people in the same household have also since tested positive for Covid – but tests are still ongoing to check if it is the same variant, so they are not included in the overall UK total of six.
PHE said officials were tracking down passengers who were on Swiss Air flight LX318 from Sao Paulo to Heathrow, via Zurich, landing on 10 February.
Sara Blackmore, South Gloucestershire’s public health director, said “fast action” by her local team meant the two cases were a “very contained situation”.
She told Radio 4’s programme that everyone in five postcode areas of South Gloucestershire was being invited to take a Covid test, even if they do not have symptoms. But she said the extra testing was a “precautionary measure” and the risk to the community was “low”.