Health officials in England are trying to trace one of six UK patients infected with a contagious coronavirus variant first found in Brazil.
Three cases of the P1 variant were picked up in Scotland and three in England earlier this month.
But one case in England remains unidentified, prompting an appeal for anyone without a result from a test on 12 or 13 February to come forward.
Labour said there has 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 in identifying the variants and mutations and therefore able to act quickly.
In England, the first two cases 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.
Testing is being ramped up in South Gloucestershire – on the outskirts of Bristol – with people living in five postcode areas invited to get tested even if they do not have symptoms. The postcodes fall within Bradley Stoke, Patchway and Little Stoke.
PHE said officials are tracking down passengers who were on Swiss Air flight LX318 from Sao Paulo to Heathrow, via Zurich, landing on 10 February.
The third case identified in England is not linked to the other two cases and the whereabouts of that person remain unknown.
PHE said the person did not complete their test registration card so follow-up details are unavailable.
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 Brazil variant is something officials are worried about because it shares similar mutations to the South Africa variant of concern.
Both have undergone genetic changes that could make them more contagious and perhaps less easy to stop with our current vaccines.
The coronavirus jabs being given to people now were designed around earlier versions of the pandemic virus, not these new variants.
Scientists believe they should still protect, although perhaps not quite as well.
Work is already under way to redesign or tweak the vaccines to make them a better match for some of these new “variants of concern”.
These updated vaccines could be ready within months, meaning the UK would have millions of doses ready to give people a booster shot before next winter to make sure the population is protected.
There will be more cases out there of new variants that have not yet been identified.
While it may not be possible to eliminate them, the determination is there to slow the spread of coronavirus, including new variants. Sticking to the social distancing rules will help.
The three Scottish residents had flown to north-east Scotland from Brazil via Paris and London, the Scottish government said.
Holyrood said the three people had tested positive while self-isolating. Other passengers who were on the same flight from London to Aberdeen are now being contacted.
Shadow home secretary, Nick Thomas-Symonds, said the news of the variant was “deeply concerning” and that it was “vital” everything was done to contain it.
“But this is further proof that the delay in introducing a hotel quarantine was reckless and the continuing refusal to put in place a comprehensive system leaves us exposed to mutations coming from overseas,” he added.
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 travellers from all countries.
Before that date, travellers arriving into the UK still needed to self-isolate in their homes for 10 days.
All travellers to the UK also need to present a negative Covid test taken a maximum of 72 hours before their departure.
It comes as the UK announced another 6,035 cases of coronavirus and reported that a further 144 people had died within 28 days of a positive test.
But a milestone in the vaccination programme was reached, with more than 20 million people having received their first dose of the vaccine.