Mixing households at Christmas could pose "substantial risks", particularly for older people more vulnerable to coronavirus, a scientist advising the government has warned.

Prof Andrew Hayward said there would be a "cost" to families getting together.

It comes as No 10 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 where generations "with high incidence of infection" socialise with older people "who currently have much lower levels of infection and are at most risk of dying" if they catch Covid-19.

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."

The UK's four nations - England, Scotland, Wales and Northern Ireland - are trying to work out a common approach to Christmas so families spread across the UK can still meet up.

Downing Street said plans for what will follow England's lockdown - which is expected to end on 2 December - and proposals to ease restrictions over Christmas would be set out next week.

A No 10 spokesman said on Thursday that ministers would keep case numbers "under review" into next week, when it will "set out more details of the next phase" after lockdown ends.

He pointed to Prime Minister Boris Johnson's previous comment that "whilst Christmas will be a little bit different from normal this year, we continue to hope to ensure that families can spend Christmas together".

Any rule change would be for a limited time, maybe just a few days, BBC health correspondent Nick Triggle has said.

But, he said, the advice was likely to urge families not to hold big gatherings and to travel by car, rather than public transport.

England is expected to return to the tier system of localised restrictions, with household mixing banned indoors in the top two tiers, when its lockdown ends.

Defence Secretary Ben Wallace said he did not want to be "the Grinch that stole Christmas" but No 10 wanted to safeguard the NHS and protect lives.

He told ITV's Good Morning Britain: "I would love all of us to be able to have a Christmas, but more than anything I want us to get through this Covid and try and get this country back to normal and I want to protect lives."

Asked if people should worry more about the health of their parents and grandparents, rather than being able to get together over Christmas, Prof Hayward added: "We're on the cusp of being able to protect those elderly people, who we love, through vaccination and it would be tragic to throw that opportunity away and waste the gains we've made during lockdown by trying to return to normality over the holidays."

His comments follow news that the University of Oxford coronavirus vaccine has shown a strong immune response in adults in their 60s and 70s - raising hopes it could protect age groups most at risk from the virus.

Prof Hayward also said that government messaging had not been consistent.

He said: "When policy is undulating between stay at home to save lives, eat out to help out, the tier system, the second lockdown and now proposals for an amnesty on social distancing - it is a highly inconsistent message."

He added that people could keep themselves and their loved ones safe by doing "relatively simple things" like avoiding crowded places and close contact with others outside their household.

His comments come after Public Health England's Dr Susan Hopkins said on Wednesday that tough restrictions might be needed before and after the holiday to allow mixing to take place.

On Wednesday, the UK recorded another 19,609 Covid cases and 529 more deaths within 28 days of a positive test, according to government figures.

Meanwhile, scientists who advised the government during the early stages of the pandemic told a BBC documentary focused on the first lockdown that they "never checked" how Covid-19 could be spread by staff working at more than one care home.