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 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.
This could mean that regions such as the South West and the north of England could also soon find themselves in Tier 4.
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.
