The government's medical adviser on Covid, Susan Hopkins, said they were working on a plan and wanted Christmas to be "as close to normal as possible".

She said tough restrictions might be needed before and after the holiday to allow mixing to take place.

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

The prime minister's official spokesman confirmed ministers were "looking at ways to ensure that people can spend time with close family over Christmas at the end of what has been an incredibly difficult year".

It comes after the Sun reported that families may be able to mix indoors for five days from Christmas Eve.

All four UK 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.

Our correspondent said any final decisions would not be made for a few weeks while health chiefs wait to see whether cases have started to come down during the lockdown in England.

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

Scientific advice indicates that for every day that measures are relaxed, five days of tighter restrictions would be needed.

The government has recorded another 19,609 Covid cases in the UK and 529 more deaths within 28 days of a positive test.

England is expected to come out of its second national lockdown on 2 December and return to the tier system of localised restrictions, with household mixing banned indoors in the top two tiers.

Speaking at a Downing Street briefing, Dr Hopkins, from Public Health England, suggested restrictions could be needed either side of Christmas if curbs were to be eased over the festive period.

She said two days of tighter restrictions would be required for every one day relaxed - although officials later clarified the advice is actually for five days.

People would need to be "very careful" about the contacts they have in the lead-up to Christmas and would have to be "responsible" and reduce contacts again after the festive period, she added.

She said she knew ministers were "working hard to develop an outline" of what the new tiers would look like after 2 December and what Christmas would look like.

The BBC has been told new tougher regional tiers could see pubs and restaurants closed entirely in areas in the top tier throughout the festive period.

Strict rules on meeting up and social distancing have meant millions of people have been unable to hug, or sometimes even see, close family for many months.

Chris, from Norfolk, said he feared this might be the last Christmas his father, who has advanced cancer, has with his three grandchildren.

"I'm not interested in Christmas as a party or celebration. All I want is one day," he told BBC Radio 5 Live.

Downing Street said Christmas would not be normal but the prime minister would look at the latest data to make decisions and an update would be given next week.

Earlier, Cabinet minister Alok Sharma said it was too early for "conclusions" but he wanted to see his family for Christmas.

Mr Sharma told BBC Breakfast people needed to keep bearing down on the infection and "do our bit".

On Tuesday, Prof Neil Ferguson, whose modelling led to the first lockdown in March, suggested extending support bubbles to up to four households to allow families to celebrate Christmas together.

This year, Christmas Eve falls on Thursday and there is a bank holiday on the following Monday, giving most workers at least a four-day break.

Prof Ferguson also warned that reopening pubs and restaurants in the run-up to Christmas would be likely to lead to rising infection levels.

'Respite from the hard slog'

What to do about Christmas is delicately poised.

On the one hand, allowing mixing over the festive period will undoubtedly lead to an increase in infections.

What is more, there are concerns the impact of lockdown will be more limited than hoped. We are yet to see infections rates start falling - although it is still early days - so there will be no final decision on Christmas yet.

But stamping down on the virus is, of course, not the be all and end all.

Providing an opportunity to meet will bring much needed respite from the hard slog of the pandemic.

But there is also a widespread recognition that even if the government bans mixing at Christmas, significant numbers of people may well ignore it.

The fear is that then starts to normalise breaking the restrictions and will make compliance worse over the rest of winter.

The expectation is that there will be some limited relaxation - in the hope that the psychological boost it will give the public and the longer-term goodwill it will engender will outweigh any cost in terms of virus spread.

That much was clear from the Downing Street briefing when government advisers admitted publicly for the first time that it may be on the cards.

But the pay-off for that could be tighter regional restrictions on hospitality in the areas with the highest rates all through the festive period.

There have been calls for a single approach from the devolved administrations in the UK about Christmas - so families who live in different nations can deal with a single set of rules.

Welsh ministers have said it could be weeks before an announcement on Covid rules is made, and warned this year's festive period would "not be like normal".

Ministers in Northern Ireland said they would do all they could to "protect" as much of Christmas as possible.

And Scotland's First Minister Nicola Sturgeon said new, stricter measures announced on Tuesday were needed, in part, to allow the possibility of people meeting up over Christmas. "We are all desperate for some normality around Christmas and I absolutely include myself in that," Ms Sturgeon said.

In other developments:

*  Prime Minister Boris Johnson answered questions at PMQs by video link as he continues to self-isolate

*  The public spending watchdog found that firms recommended by MPs, peers and ministers' offices were given priority when the government awarded £18bn worth of Covid-19 contracts - but the PM said he was "proud" of the way supplies of PPE were secured

*  Mass testing for Covid will begin in Merthyr Tydfil, one of the worst affected areas in Wales, on Saturday

*  Baroness Dido Harding, head of NHS Test and Trace, is self-isolating after receiving a notification from the NHS mobile app

*  Doctors have said lifting lockdown must be handled better this time round to avoid a surge in Covid that could overwhelm the NHS