The British Medical Journal and the Health Service Journal joined forces to say the rules should be tightened rather than allowing three households to mix from December 23 to 27.

‘We believe the government is about to blunder into another major error that will cost many lives,’ the editorial said.

‘If our political leaders fail to take swift and decisive action, they can no longer claim to be protecting the NHS.’

The health experts also warned that many Brits ‘will see the lifting of restrictions over Christmas as permission to drop their guard’.

‘The Government was too slow to introduce restrictions in the spring and again in the autumn,’ they added.

‘It should now reverse its rash decision to allow household mixing and instead extend the tiers over the five-day Christmas period in order to bring numbers down in the advance of a likely third wave.’

David Nabarro, a World Health Organisation (WHO) special envoy working on Covid-19, said the price of such a relaxation ‘could well be very high’.

Hancock refuses to say if Christmas relaxations could be scrapped

Urging people to think carefully about their plans, he told Times Radio: ‘Just ask yourself, is there any way in which you can perhaps not have the family get-togethers this year?

‘It’s much better not to do it when there’s this kind of virus about.’

The relaxation of regulations looms after the announcement that London, much of Essex and parts of Hertfordshire would be put under the strictest Tier 3 restrictions from Wednesday.

London mayor Sadiq Khan also expressed concern about the ‘Christmas window’, but denied adopting a ‘Grinch’ approach to the festive season.

He added: ‘You’re not being Grinch at all. I think what you’re doing is following the science.

‘And actually what we’ve seen across the Europe, and across the world, is those countries that are more effective at dealing with this virus having shorter more restrictive measures that help health and wealth.

‘So look at Germany and the Netherlands; they’re not for relaxing the rules over Christmas allowing three households to mix with unlimited numbers.’

Chief Secretary to the Treasury Stephen Barclay said ‘all things were kept under review’ as he urged people to show restraint at Christmas.

He told Sky News: ‘There is a balance to be struck that many families have not seen each other all year. It is important for people’s wellbeing, for their mental health. We don’t want to criminalise people for coming together as family over Christmas.

‘But, it is important that people do the minimum that is possible. So, people will be making their own judgments.’

Mr Barclay insisted the easing of restrictions over Christmas was not a ‘tier zero’ situation.

He said: ‘The position is not that we are scrapping the tiering system for five days – that we are letting people loose.

‘What we are saying is, within the family three households can come together. That’s the limit of the flexibility that is being offered.

‘I think it’s been mischaracterised as, almost, a tier zero, that we are scrapping restrictions in their entirety. That is not the case.’