Scientists advising Boris Johnson told government the number of Covid patients could exceed the January peak.
Lifting restrictions would put "enormous pressure" on an already-struggling NHS.
Sage said on December 2 there may be a higher proportion of cases in older and more vulnerable age groups during the festive period.
Professor Susan Michie warned that thee next two weeks would be crucial in ending the lockdown as planned on December 2.
Minutes from Sage meeting three weeks ago detail demand for series of measures to get on top of Covid-19.