UK travellers eligible for vaccine boosters will have to be tripled jabbed in order to take overseas trips under plans reportedly being considered by ministers.
If introduced it would see quarantine and testing reimposed for eligible groups who refuse to take up the top-up dose.
Government sources confirmed to consumer media they were looking into the proposals, which would be modelled on a system operating in Israel.
British holidaymakers are already facing travel restrictions if they wish to travel to the country or to Austria, where the authorities have begun imposing limits on the validity of vaccinations.
More: Travellers face restrictions over vaccine expiry rules
However, according to the Mail on Sunday, the government is now looking at imposing restrictions on those returning from abroad amid concerns that waning immunity over time risks them returning home with the virus.
Officials are said to be divided over how soon to implement the measures and are considering a grace period that would allow people to travel without quarantining if they had sought a booster six months after their second jab but had not yet been offered an appointment.
Sources told the Telegraph that a grace period of between one and two months was under discussion.
There is concern that holidaymakers face the prospect of being refused entry to winter ski destinations because the NHS app does not record booster jabs, the newspaper reported.
Austria and Switzerland are among a growing number of nations setting time limits on the validity of vaccinations unless travellers have boosted their immunity with follow-up jabs.
Croatia and Israel have imposed similar limits, effectively timing out travellers unless they have had their boosters.
Lithuania, the first country to introduce a digital Covid pass, has also now become the first EU country to include boosters in its app for its citizens, setting a precedent for other nations.
The Department of Health and Social Care admitted it had not designed the UK app to include boosters as “they are not required for domestic certification”.
The DHSC also pointed out that most countries did not currently require boosters as an entry requirement, although it was “aware” that some countries had begun introducing expiry dates on vaccination for travel.
A spokesman was reported as saying: “We know some countries are altering their vaccine requirements, so we continue to keep international travel certification under review to ensure British citizens can travel abroad as easily as possible.”
Airport Operators’ Association chief executive Karen Dee said the NHS app needed to “evolve” in line with changing travel rules.
She said: “The NHS Covid Pass must evolve too to enable people to prove they have had their booster vaccine where required. Other countries’ digital apps already include boosters, and we will work with the four UK governments to ensure the UK apps do so too.”
Swiss health authorities consider that a person is fully vaccinated only “for a period of 12 months from the date of complete vaccination.”
Austria has joined Italy in requiring all skiers in ski resorts this winter to be fully vaccinated, show proof of recovery or a negative Covid test to access enclosed ski lifts as well as bars and restaurants around a resort.
Everyone over 16 in Switzerland must now hold a Covid certificate – proof of Covid vaccination, recovery or a negative PCR/antigen test every three days – at hospitality venues including bars and restaurants, large-scale events and indoor sporting activities.
Israel allows visits only if the holidaymaker leaves the country no more than 180 days after their second dose.
Boosters are currently available to over-50s, health workers and the clinically vulnerable, although the scheme is expected to be extended to the over-40s early next year.
Seven out of ten of those over 80 and three in five of the over-50s in England have had their third injection, which can be administered six months after the second.
As at present, those who have not been fully vaccinated will have to self-isolate for ten full days on returning to the UK and pay for four expensive PCR tests, two before departure and two back on British soil.
It is the definition of ‘fully vaccinated’ that will be extended to include only those who have accepted the booster, according to reports.
More: Travellers face restrictions over vaccine expiry rules