The rapid spread of a new strain of coronavirus is the "worst news" of the pandemic so far, a government scientist has told Sky News.
Britons should be "very concerned" about the mutated strain of COVID-19 that is circulating in London and the South East, Professor Andrew Hayward of the government's New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) said.
Stressing the importance of how much easier this strain passes from person-to-person, he said: "This is really terrible news in terms of the pandemic.
"If the vaccine is the best news, this is the worst news we've had so far, and we really, really need to tighten down the hatches to stop the spread of this strain while vaccinating as many people as possible."
On Saturday, the new variant - named VUI-202012/01 - saw the prime minister cancel Christmas for millions of people after he was advised it is up to 70% more transmissible.
Professor Hayward, who is director of the Institute of Epidemiology and Healthcare at University College London (UCL), told Sky News that this is because the new strain results in people having a much bigger viral load.
He added that areas that have not yet seen huge caseloads of the new strain are "around three weeks behind" those put into the toughest new Tier 4 restrictions over the weekend.
Asked how the new mutation was able to thrive despite England's nationwide lockdown in November, Professor Hayward said the restrictions were "less intense" and people's attitudes to them were "less stringent".
"Worryingly, even though we had relatively strong measures that were enough to suppress the previous virus, they weren't enough to stop this one," he added.
But he said if the UK is to mitigate "many, many more deaths" as a result of VUI-202012/01, people need to reduce their contacts with others over the Christmas period.
And after a string of countries banned travel to the UK to stop the new strain getting in, Professor Hayward suggested the UK should "take its own action" to close its borders.
He disputed claims the government's action on the new strain has been "too slow", saying his NERVTAG committee first discussed it on 11 December.
"I don't think it's fair to say the government has been slow to act on this. It's more a question of how much further we need to act," he said.
Boris Johnson will chair a meeting of the government's Cobra committee later today to discuss risks of food shortages over the festive period after France banned all freight coming into the UK.
Watch our special programme tonight as Isabel Webster is joined by a panel of experts to answer your questions about the new strain of the virus, COVID Christmas rules and the new Tier 4 measures.
COVID Christmas Crisis Q&A is at 6.30pm. Send us your questions via WhatsApp on the number 07583 000853