Travellers who refuse to take a coronavirus booster shot face quarantine and tests on arrival in the UK under plans being assessed by the government.
Downing Street said it is considering a change in the definition of fully vaccinated from two shots to three, meaning those who have not had a third vaccine dose would fall foul of travel rules.
Official guidance was updated this month to say the government “is reviewing the implications and requirements of boosters for international travel certification” and “looking at whether and how booster vaccinations could be included in the NHS Covid Pass for travel”.
Britain is extending its vaccine booster programme to millions of people this winter, with the clinically vulnerable or anyone over 50 first in line.
The new measures would help to increase uptake of booster shots among the most at risk from serious illness. The government has not explained how those not yet eligible for the booster would be affected by the change.
Under current rules, unvaccinated travellers must take a PCR test three days before arriving in England, and must isolate at home for 10 days, with additional tests required on day two and eight of their return. Fully vaccinated travellers may enter the UK without any of these requirements.
Tim White, a travel consultant and data analyst, said requiring passengers to have a third vaccine makes "absolute common sense" and he expected similar measures to become the norm beyond the UK.
Countries such as Austria, Croatia and Israel now demand travellers to have had their last shot within the previous six to 12 months to ensure they are adequately protected against the coronavirus.
"If the scientific evidence shows that effectiveness wanes over time, then this third jab becomes necessary", Mr White told the National. "It's something that the UK travel industry needs to negotiate with the government. If it's going to become policy then it needs all of us to flag it up."
Figures from the NHS showed that more than 10 million people in Britain have received a vaccine top-up and authorities are hoping for an enthusiastic response from the general population.
About 30 per cent of over-80s and 40 per cent of over-50s in England are yet to receive a booster shot, the Department of Health and Social Care said.
About three million more people will also become eligible for a vaccine in the coming days.
UK Prime Minister Boris Johnson again resisted calls to move to a Plan B for tackling Covid-19, saying booster vaccines were the “key thing” as the winter months begin.
On a visit to a Hexham General Hospital in Northumberland, Mr Johnson was asked if he would heed calls from some public health officials on the need for action.
Mr Johnson said he was often presented with “different scientific advice and different scientists saying different things”.
“We study the data every day, we have reviews with NHS chiefs every day," he said.
“The key thing you want to do is to reduce those pressures, which are building, on the A&Es, on beds, by encouraging people, particularly the over-50s, to come forward and get your booster jab.”
Earlier, he said of his visit: “I’m here to look at what we’re doing to encourage people to get their booster jabs and that is the single most important thing that the government can do at the present time.
“Unfortunately, what you’ve got at the moment is a situation in which the waning of the original two jabs is starting to see too many elderly people getting into hospital.
“Sadly, the jabs do wane. We’ve done 10 million booster jabs already and it’s a very effective thing.
“It’s a wonderful thing, people get 95 per cent more protection, so I’m encouraging everybody today to go online if you’ve gone over five months, you can go online and book.”
Health Secretary Sajid Javid on Sunday said the drive to get people to have the top-up was a “national mission”.
Mr Javid said younger relatives should urge eligible parents and grandparents to have a booster and the flu vaccine.
He said that if “we all come together and play our part”, the country could “avoid a return to restrictions and enjoy Christmas”.
“We know immunity begins to wane after six months, especially for the elderly and the vulnerable, and booster vaccines will top up their protection to keep people safe over the winter," Mr Javid said.
“I strongly urge everybody who is eligible for a Covid-19 booster or flu vaccine to take up the offer as soon as you can.
“And if you haven’t yet had your first and second vaccines, it is not too late. The NHS will always be there to welcome you with open arms.”
The government said a further 62 people had died within 28 days of testing positive for Covid-19 as of Sunday, taking the UK total to 141,805.
Separate figures published by the Office for National Statistics show there have been 166,000 deaths registered in the UK where Covid-19 was mentioned on the death certificate.
As of 9am on Sunday, there had been a further 30,305 lab-confirmed cases in the UK, the government said.
