Health officials are hunting for a mystery Covid patient thought to be one of the first in the UK to have a Brazilian variant that may spread more rapidly and respond less well to vaccines.
Six individuals infected with the "variant of concern" have been detected in the UK, officials said on Sunday night.
Public Health England (PHE) has admitted it has no idea who one of them is, nor where the person was tested. Health officials have begun a scramble to try to find the person and track down hundreds of passengers on a series of connecting flights into the UK from Brazil earlier this month.
PHE is particularly concerned about this Brazilian variant, called P1, because its mutations share hallmarks with a South African variant which has been found to respond less well to Covid jabs.
It was first detected circulating in Manaus, northern Brazil, in December. Outside Brazil, it had only been picked up in travellers going to Japan until now. It is also thought to be more contagious than the original strain of Covid, similar to the Kent strain that fuelled a rise in cases in Britain in recent months.
A second, less worrying variant from Brazil, called P2, was found in the UK in January.
Anyone who undertook a Covid test on February 12 or 13 is now being asked to contact a helpline if they did not get their results or think they failed to provide contact details.
Two of the cases in England are in south Gloucestershire, where two more contacts within the same household have tested positive for Covid but await the results of gene sequencing to determine whether they have the new variant.
Health officials said they will deploy "surge testing" in five areas of the county in order to hunt down any more cases and increase the number of positive cases that are sequenced.
The third case in England is thought to be unlinked. Health officials said an investigation to find the person who tested positive for the variant is under way as they did not fill in their test registration card. They admitted they had no idea whereabouts in the country the person may be.
It might be more likely that they had undergone a home test for the virus, because officials at testing centres are supposed to check that contact details are submitted.
Officials are also seeking to find people who were on Swissair flight LX318 from Sao Paulo, Brazil which entered Heathrow via Zurich on February 10 – five days before Britain introduced hotel quarantine – carrying up to 136 passengers. One of the Gloucestershire cases was on the flight.
The developments prompted criticism of the government's policies, with Labour demanding urgent investigations into why the cases were not detected more quickly.
Three cases have also been found in Scotland and involve individuals who came back from Brazil via Paris and then London before taking a flight to Aberdeen. It is understood officials became aware of the English cases on Friday and the Scottish ones on Saturday.
On Sunday night, the Scottish government said it would track down passengers on this flight but refused to say when it was, citing patient confidentiality.
The cases are not thought to be connected to the three confirmed cases in England. All passengers on both flights should have quarantined at home for 10 days. The World Health Organisation has been informed of the cases.
Dr Susan Hopkins, PHE's strategic response director for Covid-19, said: "We have identified these cases thanks to the UK's advanced sequencing capabilities which means we are finding more variants and mutations than many other countries and are therefore able to take action quickly.
"The important thing to remember is that Covid-19, no matter what variant it is, spreads in the same way. That means the measures to stop it spreading do not change."
Yvette Cooper, who chairs the Commons home affairs committee, said: "This troubling development shows the weaknesses in the government's Covid border measures.
"The Brazil variant was first identified a month before one of these cases was brought in on February 10 and many weeks after the Prime Minister was warned that indirect flights were a problem, yet the government delayed putting stronger measures in place."
She said there were far too many holes in the current hotel quarantine system, which only covers one per cent of travellers, adding: "We need to know urgently how all these cases have arrived in the country and why they weren't prevented or picked up on arrival so that lessons can be quickly learnt and policies changed to protect the vaccine programme from further cases arriving."
Paul Charles, the chief executive of travel consultancy The PC Agency, said: "This calls into question the border measures the government claimed were watertight to prevent variants coming in. This is why only testing on arrival is going to be successful in preventing variants coming into the UK."