Prof Andrew Hayward said there would be a “cost” to families getting together.
It comes as No 10 has said proposals to ease restrictions over Christmas will be set out next week.
Scientists have said that for every day measures are eased, five days of tighter restrictions would be needed.
Prof Hayward, a professor of infectious disease epidemiology at University College London, said: “Mixing at Christmas does pose substantial risks, particularly in terms of bringing together generations with high incidence of infection with the older generations who currently have much lower levels of infection and are at most risk of dying if they catch Covid.”
Prof Hayward – a member of the government’s Scientific Advisory Group for Emergencies – told BBC Radio 4’s Today programme: “My personal view is we’re putting far too much emphasis on having a near-normal Christmas.
“We know respiratory infections peak in January so throwing fuel on the fire over Christmas can only contribute to this.”
Cases of COVID-19 rose 11% in England in the latest week, with the proportion of people testing positive staying steady, the country’s test and trace programme said on Thursday, adding that contacts reached were still near a record low percentage.
There were 167,369 people testing positive between 5 November and 11 November, up 11% on the previous week.
Of the 313,771 people identified as coming into close contact with a positive cases, 60.5% were reached, similar to the record low proportion of 59.6% hit last month.
More than 250,000 people have now died with coronavirus in the US, with the rate of new infections and hospitalisations on the rise.
Latest figures from Johns Hopkins University, which has been tracking the pandemic, showed the number of people killed by the virus in the country is now 250,485.
It comes as the pandemic shatters more records nationwide, with the daily count of new coronavirus cases running at close to 160,000 on average – a jump of 80% in the last two weeks.
Starting Thursday, New York’s public schools will be closed to combat a rise in coronavirus cases, its mayor announced Wednesday, dealing a blow to the city’s recovery.
Bill de Blasio said the schools would be temporarily shut again “out of an abundance of caution” after the city recorded a seven-day average positivity rate of three percent.
“We must fight back the second wave of Covid-19,” he wrote on Twitter.
The three-percent threshold had been agreed between the city government and teaching unions when New York’s schools began reopening in September.