Covid-19 cases rose by almost a third over the festive period, reaching 70,000 a day by the beginning of January according to a major study.
This comes as an Office for National Statistics (ONS) survey suggests just under half (44%) of British adults said they had formed a Christmas bubble.
Up to three households were allowed to mix in much of the country on 25 December.
The ZOE Covid Symptom Study found the epidemic is growing throughout the UK.
Researchers estimate the virus’s reproduction or R number is currently 1.2, which chimes with official government estimates of 1.1-1.3.
It is growing fastest in Scotland and, in England, London has the most severe epidemic with the highest number of cases.
Of those responding to the ONS survey, about a fifth said they had found it difficult to follow the Christmas rules.
And many gave the fact that they had already made plans as the reason.
Rules, which were set to allow everyone in the UK to mix in a five-day window, were changed at the last minute, on 19 December.
In England, people living in Tiers 1-3 were allowed to form a one-day Christmas bubble with a maximum of two other households.
Those in Tier 4, including about 10 million people in Greater London, were not permitted to mix at all.
Mixing was permitted in Scotland and Wales for Christmas Day only.