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 is understood to have used a home testing kit but did not complete a registration form – 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.
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.
The hotel quarantine rule came into force on 15 February. It means that travellers coming to England from 33 countries – including Brazil – must pay to quarantine in a hotel for 10 days, while in Scotland the rule applies to international travellers from all countries.
Before that date, travellers arriving into the UK still needed to self-isolate in their homes for 10 days.
But Vaccines Minister Nadhim Zahawi said it was not known whether or not the unidentified person had recently been abroad, so it was not clear if they would have been self-isolating while they were infected.
“We need to find them as soon as possible,” he told BBC Breakfast, adding that PHE was working with the postal service to try to locate them.
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.