Video report by ITV News Correspondent Romilly Weeks
Health Secretary Matt Hancock has pleaded with anyone who may be the missing carrier of a Covid-19 variant to "get in touch" with officials, so the spread of the strain can be contained.
Anyone in England, Wales or Northern Ireland should contact 119. Anyone in Scotland should contact 0300 303 2713.
The government is keen to contain all cases of the new strain, first identified in Brazil, because it is thought antibodies from previous infection have a "reduced impact".
It is also unclear whether the "variant of concern" has mutated to become resistant to coronavirus vaccines, said Dr Susan Hopkins, strategic response director at Public Health England.
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Six cases of the variant have been identified in the UK, however officials have been unable to locate one carrier who has gone under the radar because the person did not complete their test registration card.
Dr Hopkins told the press conference: "We are tracking it very closely because it shares some important mutations with the variants first identified in South Africa."
She added: "These and other mutations are associated with reduced impact of antibodies against the virus in laboratory experiments.
"The current vaccines have not yet been studied against this variant and we will need to await further clinical and trial data to understand the vaccine effectiveness against this variant."
According to Dr Hopkins it is "extremely rare" for a case to go missing due to someone failing to complete their test registration form.
Health officials are currently "in the process of doing everything we can to find this individual" she said.
Passengers on board Swiss Air flight LX 318 on February 10, which flew from Zurich to London, are also being urged to contact the NHS if they were among the 10% of those not tested.
Mr Hancock defended the government's border arrangements following the detection of the cases of the Manaus variant, which was imported to the UK just five days before mandatory hotel quarantine was enforced for arrivals from South America and southern Africa.
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He said home quarantine measures were already in place and travel restrictions on Brazil had been imposed before the hotel policy was implemented.
Mr Hancock said: "All the evidence is that the five cases that we know about followed those quarantine rules and that, I hope, is very reassuring to people.
"There is no evidence that the sixth case did not follow those quarantine rules - we need to obviously get in contact with the person in question."
The test was taken on February 12 or 13 and "we haven't seen any further knock-on transmissions in the data".
A member of the government's Scientific Advisory Group for Emergencies (Sage) warned earlier today the emergence of a Brazilian variant of coronavirus in the UK could mean the planned roadmap out of England's lockdown is delayed.
Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine told the BBC: "There are going to be challenges on the way - and there is always a risk that we might have to go backwards, and that's what nobody wants to do is to actually open up and then have to close down again."
Experts have expressed concern about the variant - which was first detected in the city of Manaus - may be able to get around antibodies.
Danny Altmann, professor of immunology at Imperial College, told Times Radio Manaus had one of the highest levels of immunity in the world due to high infection levels earlier in the pandemic.
But he pointed out the city was currently suffering under an "enormous" wave of reinfections.
He said: "So, if you put two and two together their assumption is that's because the new variant is breaking through those antibodies. But if that hadn't really been proven yet, it looks likely."It comes after the announcement that the UK has vaccinated 20,275,451 people for Covid-19 (around 30% of the population) with 815,816 of those having had a second dose.
Earlier on Monday it was announced the number of new daily coronavirus cases had dropped below 6,000 for the first time since September.
And a further 104 deaths were reported - the lowest daily figure since October - bringing the UK's official Covid-19 death toll to 122,953.
Mr Hancock said the number of new coronavirus cases was falling but the rate of decline has slowed, urging people to "keep sticking to the rules, let's not blow it now".
But the number of admissions to hospital was falling faster than that of cases - particularly among the older age groups who were vaccinated first.
"This is a sign that the vaccine is working," he said.
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