The new variant of Covid-19 is "hugely" more transmissible than the virus's previous version, a study has found.
It concludes the new variant increases the Reproduction or R number by between 0.4 and 0.7.
The UK's latest R number has been estimated at between 1.1 and 1.3. It needs to be below 1.0 for the number of cases to start falling.
Prof Axel Gandy of London's Imperial College said the differences between the viruses types was "quite extreme".
"There is a huge difference in how easily the variant virus spreads," he told BBC News. "This is the most serious change in the virus since the epidemic began," he added.
The Imperial College study suggests transmission of the new variant tripled during England's November lockdown while the previous version was reduced by a third.
Cases of Covid-19 have begun to increase rapidly during the second spike, and the number of cases recorded in a single day reached a new high on Thursday.
Early results indicated that the virus was spreading more quickly among under-20s, particularly among secondary school age children.
But the very latest data indicates that it was spreading quickly across all age groups, according to Prof Gandy who was a member of the research team.
"One possible explanation is that the early data was collected during the time of the November lockdown where schools were open and the activities of the adult population were more restricted. We are seeing now that the new virus has increased infectiousness across all age groups."
Tougher restrictions
Prof Jim Naismith, of Oxford University, said he believed that the new findings indicated that even tougher restrictions would soon be needed.
"The data from Imperial represent the best analysis to date and imply that the measures we have employed to date, would - with the new virus - fail to reduce the R number to below 1.
"In simpler terms, unless we do something different the new virus strain is going to continue to spread, more infections, more hospitalisations and more deaths."
The R number is the average number of people an infected person infects. If it is above 1 the epidemic is growing.
BBC
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