People are being urged to rethink their Christmas plans after the Prime Minister warned that the coronavirus situation in the country is worsening.
Mr Johnson was speaking at a press conference this afternoon alongside Chief Medical Officer Chris Whitty.
He confirmed that his government's plan to allow people to meet each other in extended family bubbles over a 5-day period at Christmas will remain - but implored people to think again about their plans.
He said: "'We will win our long struggle against this virus, which makes it all the more important that we hold our nerve.
"We are in a race to protect ourselves while doing everything we can to keep the virus under control - but we are already seeing worrying rises in some parts of the country.
"We come to this moment, a time of year of immense emotional and spiritual importance, but also a moment for us to exercise extreme caution.
"This virus spreads invisibly, even amongst those who don't have symptoms and it is vital that everyone exercises the greatest personal responsibility."
Mr Johnson accepted that the virus situation in the country is now 'more challenging' than when he set the festive rules several weeks ago.
He said: "While it would not be right to criminalise people who have made plans, we are collectively asking you to think hard and in detail about the days ahead and whether you can do more to protect each other.
"A smaller Christmas is going to be a safer Christmas and a safer Christmas will be a safer Christmas."
He said the rules allowing 3 households to mix over 5 days are "maximums rather than targets."
The Prime Minister urged people not to travel from areas of high virus prevalence to areas of low prevalence if possible and to avoid overnight stays.
He added: "Remember a vaccine is on the way and our aim is to inoculate everyone who is extremely vulnerable or elderly in the early months of the new year."
"Whatever your plans for Christmas, please think carefully about avoiding the Boxing Day sales and no one should be gathering in large groups to see in the New Year."
"Have yourselves a merry little Christmas and I'm afraid this year I do mean little - I'm sure that next year Christmas will be normal for every family."
Chief Medical Officer Chris Whitty said the fact that vaccines are now on the way made it even more important to take great care now.
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He said: "It is very important to situate the actions people will take over the next few weeks with the fact that we have a vaccine that will protect the most vulnerable people from the virus over the next few months.
"As the vaccine is rolled out across the general population, the risk of coronavirus circulating will begin to go down.
"Anyone who becomes infected this Christmas would have been protected in the very near future.
"It is very important that people think about that when they make their decisions in the next few weeks."
He added: "keep it small, keep it short, keep it local and think about the most vulnerable people."
