In recent days there have been widespread shortages of lateral flow and PCR tests for delivery on the government's portal, with no PCR slots available across the UK this morning.

Health Secretary Sajid Javid says the government will triple the supply of lateral flow tests to 300 million per month by February, but added that as current shortages persist "we expect to need to constrain the system at certain points over the next two weeks".

Professor Ravi Gupta, clinical microbiologist at the University of Cambridge, told Sky News the lack of available tests was "a great concern".

"Because, of course, we already have so many cases that therefore the probability of an individual being in a social environment on New Year's Eve and being positive, even if they have potentially tested negative in the last 24 hours, is fairly high," he said.

"The transmissions are going to be significant and, of course, the availability of tests... of lateral flow testing... is going to compound that problem.

"So I'm personally very worried about what's going to happen next year."

Prof Gupta's advice to people planning to celebrate on New Year's Eve was to try to test before attending an event, make sure others around them are testing, wear good quality masks and to "keep things as small as possible".

Professor Andrew Hayward, who sits on the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), which advises the government on COVID, accused ministers of giving "mixed messaging" ahead of New Year's Eve.

Nightclubs have been ordered to close in Scotland and Wales, while Edinburgh's traditional Hogmanay street party has been cancelled and Scots are being encouraged to limit gatherings to no more than three households.

But, in England, Prime Minister Boris Johnson has not introduced any further restrictions for New Year's Eve, even though he has urged people to celebrate in a "cautious and sensible way" and suggested they test themselves prior to meeting with others.

Scots have been urged not to travel to England for New Year's Eve celebrations to get around stricter COVID rules north of the border.

Meanwhile, one business owner in Chester told Sky News he was expecting people in Wales to travel across the border to enjoy a night out at his venue.

Prof Hayward, speaking to Sky News in a personal capacity, said: "Going to an indoor party is the opposite of being cautious so it feels like quite mixed messaging to me. But I do think if you are going, it is important you do take a test.

"I think it's also going to be important, if you've been to one of those parties, particularly before you then go and visit relatives or elderly people, to be taking tests then."

Prof Hayward added that having an availability of COVID tests would be "critical" to continuing to allow people to socialise as normal.

He also suggested it was "odd" that new Nightingale "surge hubs" are to be set up in England to increase NHS capacity, at the same time that people were not being discouraged from having New Year's Eve parties.

"I find it odd really that we're in a situation where we're worried enough about the wave of Omicron to build new Nightingale capacity," he said.

"But we're not worried to suggest to people that New Year's Eve parties are not a good idea.

"New Year's Eve parties, especially if you are in an indoor crowded environment with lots of other people who you don't know, it's difficult to imagine a better way of spreading COVID and giving this Omicron variant a big boost."

The warnings from scientists came as Mr Javid admitted there was "unprecedented demand" for both PCR and lateral flow tests (LFDs), which has "inevitably placed strain on the testing system".

In a letter to MPs, the health secretary said: "In light of the huge demand for LFDs seen over the last three weeks, we expect to need to constrain the system at certain points over the next two weeks to manage supply over the course of each day, with new tranches of supply released regularly throughout each day."

Mr Javid promised that capacity would be ramped up next year, with the government buying millions more lateral flow tests, bringing in new products and accelerating deployment to the public.

"We are also doubling our total delivery capacity with Royal Mail to 900,000 test packs and PCR tests a day," he added.

"We are tripling the supply of LFDs in January and February from our pre-Omicron plan of 100 million to 300 million per month."

Professor Martin Marshall, chairman of the Royal College of GPs, said the Omicron variant was still "spreading like wildfire" and causing "massive problems" for doctors' surgeries.

He told Times Radio that GPs are seeing increasing numbers of patients with COVID-19 and increased numbers of staff are having to self-isolate.