Boris Johnson has raised the prospect of Britons soon needing a booster jab to be considered “fully vaccinated” when going abroad or for self-isolation rules.

Speaking at a Downing Street news conference, the prime minister revealed plans to add evidence of a vaccine top-up to the NHS COVID pass.

He also said Britons would find “life easier in all kinds of ways”, including for foreign travel, with a booster dose.

Following an earlier announcement that the rollout of COVID booster shots will now be extended to all over-40s, Mr Johnson highlighted increased vaccination rates as a means of avoiding the reintroduction of domestic restrictions.

The prime minister warned that “storm clouds are gathering over the continent” with a “new wave” of COVID cases sweeping westwards through Europe, prompting some countries to reimpose coronavirus measures.

He said that “countries with lower vaccination rates have tended to see bigger surges in infection, and in turn been forced to respond with harsher measures”.

“We don’t yet know the extent to which this new wave will wash up on our shores, but history shows we cannot afford to be complacent,” Mr Johnson added.

The prime minister said he saw “nothing in the data at the moment” that would force the government to move to “Plan B” of its winter contingency planning for the NHS, which could see the reintroduction of mandatory face masks or the return of work from home guidance.

But he warned that ministers “cannot rule anything out” when asked about the prospect of a Christmas lockdown.

Mr Johnson said the “most important thing people can do” to prevent the reimposition of restrictions was to “get the boosters”.

Asked if evidence of a booster jab would be added to the NHS COVID pass, Mr Johnson replied: “I think 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 from 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 on foreign travel.

“So I would just say, if you’re thinking about that, then this is yet another reason to get it done.”

The prime minister later suggested that, at some point, only those who have had a booster jab – which for most people is a third vaccine dose – would be considered “fully vaccinated”.

“On boosters, 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,” he said.

“We will have to adjust our concept of what constitutes a full vaccination to take account of that. I think that is increasingly obvious.

“The booster massively increases your protection, it takes it right back up to over 90%. As we can see from what’s happening, the [protections afforded by] two jabs sadly do start to wane.

“We’ve got to be responsible and we’ve got to reflect that fact in the way we measure what constitutes full vaccination.”

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The prime minister said that, so far, around 12.6m people across the UK have had a booster jab.

In England, this includes three-quarters of all people aged over-70 and 80% of eligible older people in care homes.

It would be an “utter tragedy” if people who had had two vaccine doses “ended up becoming seriously ill or even losing their lives because they allowed their immunity to wane”, Mr Johnson added.