Boris Johnson says he plans to change the definition of being fully vaccinated against coronavirus.
The prime minister indicated on Monday that changes will need to be implemented so that people will only be considered fully vaccinated if they have the COVID-19 booster jab.
As a result, the third jab could be added to the NHS COVID Pass and may be needed to travel abroad without restrictions.
During a briefing from Downing Street, Johnson said: “It’s very clear that getting three jabs – getting your booster – will become an important fact and it will make life easier for you in all sorts of ways.
“We will have to adjust our concept of what constitutes a full vaccination to take account of that.
“And I think that is increasingly obvious.”
It was announced on Monday that the booster programme will be extended to include people aged 40 and over.
Should the Joint Committee on Vaccination and Immunisation (JCVI) continue to recommend boosters for all adults six months after their second jab, then three jabs could be required in England to be fully vaccinated by spring.
Watch: Boris Johnson pleads for Britons to take boosters to avoid 'tragedy'
Currently, the NHS COVID Pass in England lets someone show the details of their coronavirus vaccine status domestically and when travelling abroad to some countries or territories.
Johnson said: “I think that we will be making plans to add the booster dose to the NHS COVID travel pass.
“But again, I think what the general lesson is for anybody who wants to travel, you can see that getting fully vaccinated with a booster is going to be something that will, on the whole, make your life easier in all kinds of ways including foreign travel.
“So I would just say, if you’re thinking about that, that this is yet another reason to get it done.
“The booster massively increases your protection. It takes it right back up to over 90%.
“And as we can see from what’s happening, the two jabs sadly do start to wane. So we’ve got to be responsible, and we’ve got to reflect that fact in the way we measure what constitutes full vaccination.”
Rory Boland, Which? travel editor, said: “It’s welcome news that the government is looking to add evidence of booster vaccinations to the NHS app.
“This issue has some urgency as other countries have already introduced the requirement to have had a booster jab as they limit the validity of vaccines, but with UK residents currently unable to provide evidence that they have done so they risk being turned away from the plane on their next trip.”
How does being double-jabbed impact everyday life?
Many countries abroad insist that visitors are fully vaccinated before entering. Some countries require proof of both jabs to attend large venues, or go to bars and restaurants.
According to government guidance, if you are fully vaccinated (currently, have had two coronavirus jabs), before travelling back to England from abroad you must book and pay for a COVID-19 test (lateral flow or PCR), to be taken before the end of day 2 in England.
You must also fill in a passenger locator form, to be completed in the 48 hours before you arrive in England.
You will need to enter your COVID-19 test booking reference number on your passenger locator form.
To qualify as fully vaccinated, you must have had your final dose of the vaccine at least 14 days before you arrive in England.
If you are not fully vaccinated, before travelling to England you must take a COVID-19 test in the three days before travel.
You must also book and pay for Day 2 and Day 8 COVID-19 PCR tests to be taken after arrival in England.
In addition, you must complete a passenger locator form in the 48 hours before you arrive in England.
If you are not fully vaccinated, when you arrive in England you must quarantine for 10 full days and take your COVID-19 PCR tests.
If someone in your household tests positive for COVID-19, you are not required to self-isolate if you are fully vaccinated.
If you have only received one dose of a COVID-19 vaccine, you will still be required to self-isolate, according to government advice.
Businesses in England can use the NHS COVID Pass, the government says.
Use of the pass is voluntary for individual organisations in England, but the government encourages it to be used in events with a large number of people for a sustained period of time.
This is likely to include nightclubs and music venues, business events and festivals, as well as sporting events.
In Wales, people now need to show their NHS COVID Pass to gain entry to cinemas, theatres, concert halls and nightclubs.
Scotland has its own NHS COVID pass to prove you are double jabbed, required to enter nightclubs and large events.
Northern Ireland does not have a mandatory vaccine passport scheme, although a Cert Check NI app has been launched for voluntary use at entertainment venues.
Watch: COVID-19 vaccine booster programme extended to over-40s