Boris Johnson has warned the UK faces the risk of a Covid "blizzard" from the east as infections rise in parts of Europe.
The Prime Minister urged people to get their boosters to see off the threat of a winter surge in the wake of new guidance advising over-40s to get a third dose.
Mr Johnson warned of a "storm of infection" in parts of Europe - but said the Government did not believe there was a need for tougher restrictions at home.
And he told Brits that getting a booster "could make all the difference to winter, to Christmas".
Speaking at a vaccination centre in east London, the PM said: "We don't see anything in the data at the moment to suggest that we need to go to Plan B, we're sticking with Plan A.
"But what we certainly have got to recognise is there is a storm of infection out there in parts of Europe, you can see those numbers ticking up very sharply in some of our continental friends.
"And we've just got to recognise that there is always a risk that a blizzard could come from the east again, as the months get colder.
"The best protection for our country is for everybody to go forward and get that booster."
It comes as the Joint Committee on Vaccination and Immunisation (JCVI) issued new advice saying people aged 40 to 49-years-old should be given booster jabs.
The vaccine experts said the over-40s should be offered Pfizer or Moderna jabs six months after their second dose - regardless of which vaccine they had originally.
The JCVI also changed its advice for 16 to 17-year-olds, who are now told to get a second Pfizer jab at least 12 weeks after the first.
Mr Johnson said: "When you look at what's happening in the pandemic at the moment... there are people in ITU, in intensive care, who are suffering badly from Covid, but they are all the unvaccinated.
"And what's happening is that if you can get your booster, then your immunity goes right back up to 95%.
"So far, we've got 75% of everybody over 70 getting a booster, that's a huge number of people, but it's that further 25% that will make all the difference to the winter, to Christmas, to our plans going forward, because it's that extra level of protection that we really need."
Earlier, Tory chairman Oliver Dowden said it was up to the public whether new controls would need to be imposed over the winter months.
He said the vaccination programme offers the best assurance that further Covid-19 restrictions will not be needed over Christmas.
He told Sky News: "It is in our hands. If you get the booster when the call comes that is the biggest wall of defence that we have against Covid."
