The relaxation of coronavirus restrictions over Christmas will increase infections "potentially by a large amount", the government's scientific advisers have warned.
The Scientific Advisory Group for Emergencies (SAGE) said the prevalence of COVID-19 "could easily double" over the festive period with "substantial mixing" of people.
The warning comes in a document called "notes on festive period", which was written more than a week ago but published for the first time on Friday.
"Any relaxation over (the) festive period will result in increased transmission and increased prevalence, potentially by a large amount," the paper says.
"A parallel can be drawn, albeit on a different scale, between the return of students in autumn and people from different households mixing intensively over Christmas."
SAGE says that "substantial mixing of people over a short period of time, especially those who do not make contact regularly during a month represents a significant risk for wide-spread transmission".