© AP Photo / Christophe Ena
The third jab could become a requirement to avoid travel restrictions in a move which may further drive take-up of boosters.
Official guidance was updated earlier 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".
A government source told the Mail on Sunday: "This is not going to happen immediately - but happen it will."
The controversial move would change the definition of 'fully vaccinated' from having two jabs to being triple jabbed.
So far almost 10 million people in the UK have received a top-up jab, but around 30% of over-80s and 40% of over-50s in England are yet to receive a booster shot of vaccine.
Producing a negative Covid-19 test will not be enough to bypass the rules, according to the paper.
It added that officials are divided over how soon to implement the measures and are discussing a grace period to allow people to continue to travel without quarantine if they have sought a booster six months after their second jab but have not yet been offered an appointment.
Three million more people in England are being invited to have their coronavirus booster jabs next week.
People over 50 and those most at risk from Covid-19 are eligible for a booster six months after their second jab.
Health Secretary Sajid Javid said younger relatives should urge eligible parents and grandparents to take up the offer of a booster and the flu vaccine.
He said that if "we all come together and play our part" then the country can "avoid a return to restrictions and enjoy Christmas".
"Almost 10 million people in the UK have received their Covid-19 booster and third jabs, a phenomenal achievement in under two months," Mr Javid said.
"As we approach this milestone, I want to thank those who have come forward and urge everybody across the nation to get vaccinated, get protected and get boosted.
"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.
"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.
"This truly is a national mission.
"If we all come together and play our part, we can get through this challenging winter, avoid a return to restrictions and enjoy Christmas."
Comment: That's what they say every time they are about to shift the goalposts...
NHS England said eight million people were vaccinated by Saturday out of 12 million people that have been invited.
By next week, 16 million invitations for a booster will have been sent since the NHS booster programme began.
Dr Nikki Kanani, deputy lead for the NHS vaccination programme in England, said: "Thanks to the hard work and non-stop efforts of NHS staff, the NHS Covid-19 vaccination programme has now vaccinated more than eight million people ensuring they are protected ahead of winter.
"The NHS has invited millions of people who are eligible for a booster vaccine, and with hundreds of walk-in sites or appointments available online if you prefer, it is quick and easy to get vaccinated, I've had my booster and I would encourage anyone who is eligible to come forward as soon as possible to protect you and your loved ones."
Text invites will come from NHSvaccine and will include a link to the NHS website, enabling anyone eligible to book an appointment at a convenient site nearby.
People who have not received their invitation can book an appointment on the NHS website or call 119.
Eligible people can also attend local walk-in sites without an appointment and can find their nearest walk-in through the online site finder.
More than 88 million vaccines have already been delivered and nine in 10 adults have had their first dose since Margaret Keenan received the first jab outside of a clinical trial in Coventry in December 2020.
Or maybe 30 per cent ?