Professor Peter Openshaw, who sits on the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said the conditions at a New Year’s Eve gathering were “perfect” for spreading coronavirus.
Asked about the prospect of untested people mixing due to a shortage of lateral flow devices, he told BBC Radio 4's Today on Thursday: “I think it’s very worrying indeed.
“We know the situations in which transmission happens and fortunately I don't think we are facing the sort of lockdown that was necessary in order to cope in the very earliest part of this year.
“But we do know that crowding together in poorly ventilated spaces, particularly if you are shouting over loud music and so on, is absolutely perfect in terms of transmitting this very, very highly transmissible virus.”
That means New Year’s Eve parties are allowed to go ahead in England, with nightclubs allowed to open for partygoers and no limits on numbers in pubs.
Ministers have advised people to be “cautious” as they ring in 2022, urging them to take lateral flow tests before heading out.
But the UK is in the grip of a shortage of testing kits due to unprecedented demand fuelled by that very guidance. A survey of pharmacists in London conducted by the Standard on Wednesday found that three-quarters had run out of lateral flows.
It comes as the Omicron variant continues to surge, with a record 183,037 Covid-19 cases reported in the UK on Wednesday - over 53,000 more than the previous highest figure registered just a day earlier.
The rise, sparked by the highly transmissible Omicron variant of the virus, also coincides with an increase in the number of patients in hospital who have tested positive for coronavirus.
However Mr Johnson said the high number of people who had received Covid booster shots, which now stands at more than 33 million or about 58 per cent of the population aged over 12 - meant there was no need for new rules ahead of New Year celebrations.
“Enjoy New Year sensibly and cautiously,” he said on Wednesday. “We’ve got cases certainly going up, we've got a lot of cases of Omicron, but on the other hand we can see the data about the relative mildness of Omicron.”
Hospitalisations have also been rising sharply and according to official figures, there were 10,462 people in hospital in England with Covid-19 on Wednesday, up by 916 from the previous day.
Of these, 771 needed to be in mechanical ventilation beds, a figure that has changed little during December.