Mr Johnson said “continuing uncertainty” about the severity of the Omicron variant and hospital admission rates meant he did not believe there was enough evidence at the moment to justify stricter measures.
But he said the situation remained ‘finely balanced’ and added that people should ‘exercise caution’.
In a video message updating the public on the latest position, the Prime Minister said: “There is no doubt that Omicron continues to surge with a speed unlike anything we’ve seen before.
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“The situation remains extremely difficult but I also recognise that people have been waiting to hear whether their Christmas plans are going to be affected.
“So what I can say tonight is that naturally we can’t rule out any further measures after Christmas – and we’re going to keep a constant eye on the data, and we’ll do whatever it takes to protect public health.
“But in view of the continuing uncertainty about several things – the severity of Omicron, uncertainty about the hospitalisation rate or the impact of the vaccine rollout or the boosters, we don’t think today that there is enough evidence to justify any tougher measures before Christmas.
“We continue to monitor Omicron very closely and if the situation deteriorates we will be ready to take action if needed.
“What this means is that people can go ahead with their Christmas plans but the situation remains finely balanced and I would urge everyone to exercise caution, to keep protecting yourselves and your loved ones, especially the vulnerable.
“And remember to keep following the guidance – wear a mask indoors when required to do so, keep fresh air circulating, and take a test before you visit elderly or vulnerable relatives.”
He went on to encourage people to “drop everything” if they have yet to get a vaccine.
Mr Johnson’s comments come after Chancellor Rishi Sunak offered a £1billion support package to hospitality and leisure businesses hit by Covid restrictions amid concerns over the high transmission of Omicron.