A government minister has said the travel rules were tightened to stop new variants arriving from abroad just as a Sage member warned that the new Brazilian strain is probably already in the UK.
Aviation minister Robert Courts insisted on Saturday morning that the government has a "very strong" package of measures in place to protect the public from any new coronavirus variants.
Courts told BBC Radio 4's Today programme they are "toughening up already tough requirements" to ensure that new variants do not arrive from abroad while the vaccine is rolled out.
He added a "total ban" on travel to the UK would not be right, and that pre-departure testing, passenger locator forms and the quarantine period make the system "robust".
It comes after the government banned flights from South America, Portugal and Cape Verde on Thursday in response to the emergence of the new variants, having previously banned travel from South Africa because of a new Covid strain.
In addition, all quarantine-free travel into the UK will be suspended on Monday in a bid to keep out other variants.
But eight genomically confirmed cases of the Brazil variant have already been identified in the UK.
The second, which has been detected in Manaus and in travellers arriving in Japan, has not been detected in the UK so far.
But Professor John Edmunds, a member of the government's Scientific Advisory Group for Emergencies (Sage), said it would be “unusual” if the second was not already present.
The new policy means arrivals from every destination will need to self-isolate for 10 days or receive a negative result from a coronavirus test taken at least five days after they enter the UK.
Prof Edmunds told BBC Radio 4's Today programme: "In terms of the South African one, we had imported cases already by the time we put in additional restrictions for South African travellers.
"For the Brazilian one... I don't think there is evidence that we've imported cases of the Manaus strain, as far as I'm aware at least, but it is likely that we probably have quite honestly.
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"We are one of the most connected countries in the world so I would find it unusual if we hadn't imported some cases into the UK."
Labour accused the government of "closing the door after the horse has bolted", saying the announcement was too late to have stopped the arrival of "worrying" strains.
Shadow home secretary Nick Thomas-Symonds told ministers to get a "comprehensive plan and to act in a proper, strategic way, not in the short-term chaotic way we've seen over the past twelve months".
The Airport Operators Association told the BBC that there was "only so long" before airports might have to close temporarily to save costs and called for more support.
Tim Alderslade, chief executive of Airlines UK, also told the broadcaster: "We've had no revenue now effectively for 12 months, give or take a few months in the summer last year. If we're going to have an aviation sector coming out of this we need to open up in the summer.”
Abta, a trade association of travel agents and tour operators, told the BBC that the government should provide support "as a matter of urgency" both for the jobs and businesses at risk.
Meanwhile, Professor Andrew Pollard, director of the Oxford Vaccine Group, said there would be lots of new coronavirus variants this year but the current vaccines should protect against the strains circulating in the UK.
He said that new variants were being detected early, and stressed: "If indeed we do need to make new vaccines we will be able to stand those up really quickly."
More than 3.2 million people have now received the first dose of a Covid-19 vaccine in the UK - around double the number compared to last week.
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