Summer holidays abroad are “extremely unlikely” for Britons despite almost half the adult UK population having received at least one dose of a Covid-19 vaccine.
Scientists said Britain would be at "real risk" if widespread international travel occurred in July and August.
Half of England's adult population has received at least one shot of the vaccine, with Wales and Scotland not far behind.
Britain’s relatively smooth-running inoculation programme had raised hopes of a return to normality, but members of Sage, the panel of scientists that advises the UK government, said such optimism was premature.
“I think that international travel this summer is, for the average holidaymaker, sadly I think, extremely unlikely,” said Dr Mike Tildesley, a member of the Scientific Pandemic Influenza Modelling group.
“I think we are running a real risk if we do start to have lots of people going overseas in July, for instance, and August because of the potential for bringing more of these new variants back into the country.
“What is really dangerous is if we jeopardise our vaccination campaign by having these variants, where the vaccines don’t work as effectively, spreading more rapidly.”
Andrew Hayward, another adviser to the government, said travel to countries currently reporting high caseloads was “unlikely” to be allowed.
Parts of Europe, including France, Italy and Germany, fear they are experiencing the third wave of the pandemic.
“I think it’s unlikely that we would want to encourage travel to those countries whilst they have high levels of infection,” Prof Hayward said.
“I suppose one of the more worrying things about this resurgence is that in some parts of Europe the South African variant is beginning to creep up to higher levels.”
There is also widespread doubt in tourism destinations about how the summer will unfold.
One third of all hotels may not even open in Portugal, while Croatia expects this year’s overnight stays to be only 60 per cent of the record set in 2019.
“2021 seems to be worse than last year; there are no pre-bookings,” said Lysandros Tsilidis, president of Greece’s Association of Tourist and Travel Agencies.
The EU is working to reduce the obstacles for sun-seekers, unveiling a plan for a digital pass that will ease travel for those who have been vaccinated, recovered from the virus or had a recent negative test.
In England, the latest statistics show 22,337,590 people – or 50.5 per cent of the adult population – had been given a first dose of Covid vaccination.
Wales is at the equivalent of 47.7 per cent of adults and Scotland at 46.6 per cent.
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Prime Minister Boris Johnson received his first dose of the vaccine on Friday at St Thomas’ Hospital in London, across the River Thames from the Houses of Parliament.