Boris Johnson has urged people to get vaccinated, as he warned of a Covid "storm cloud" approaching from Europe.
The Prime Minister, speaking at a Downing Street press conference, said a new wave of the virus was sweeping across the continent and officials did "not yet know the extent to which it will wash up on our shores."
Mr Johnson said to avoid a return of lockdowns or Covid restrictions "we must all get vaccinated as soon as we are eligible".
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The comments came as Joint Committee on Vaccination and Immunisation (JCVI) recommended that the booster programme should be extended to include people aged 40 to 49.
Second doses for 16 and 17-year-olds were also been approved after the JCVI said this group should be offered a second jab of the Pfizer/BioNTech jab 12 weeks after they received the first.
Mr Johnson said: "The storm clouds that are gathering over parts of the continent, a new wave of Covid has steadily swept through Central Europe and is now affecting our nearest neighbours in Western Europe.
"Our friends on the continent have been forced to respond with various degrees of new restrictions from full lockdowns, to lockdowns for the unvaccinated to restrictions on business opening hours to restrictions on social gatherings.
"We don't yet know the extent to which this new wave will wash up on our shores, but history shows that we cannot afford to be complacent.
"Indeed, in recent days, there have been rising cases here in the UK so we must remain vigilant, because there is one lesson we can draw from the current situation in Europe.
"Those countries with lower vaccination rates have tended to see bigger surges in infection and in turn being forced to respond with harsher measures.
"While those countries with higher vaccination rates have so far, fared better. This shows us once again that if you want to control the epidemic here in the UK and if we want to avoid new restrictions on our daily lives, we must all get vaccinated as soon as we are eligible."
Asked whether there were any plans for a Christmas lockdown, Mr Johnson said there was nothing in the data that indicated a lockdown would be required but added: "Clearly we cannot rule anything out".
