The new, more infectious strain of coronavirus transmission rate is "likely to have tripled" - even during England's November lockdown.
Axel Gandy, chair in statistics at Imperial College London, said the high rate of infection in the new coronavirus variant meant that transmission of the disease was likely to have increased significantly.
It comes after the MRC Centre for Global Infectious Disease Analysis at Imperial confirmed the variant does spread quicker.
Professor Gandy told BBC Radio 4's Today programme on Friday: "Overall, we've been able to determine that the new variant increases the reproduction number - so that's the number of people infected on average per infected person in the future - by about 0.4 to 0.7.
"That doesn't sound like much but the difference is quite extreme.
"Under the November lockdown conditions, the reproduction rate was in the region of 0.9 and with the old variant that would mean that over two to three weeks, the cases would decrease by about 30%, whereas in the new variant they would triple.
"The number of cases would increase by a factor of three, so there is a huge difference in how easily this virus spreads."
He added that since the November lockdown had been eased, it did not look like there was evidence to suggest children of school-going age were more likely to carry the new variant.
The news comes as hospitals in London and the south faced difficult challenges in recent days.
The Royal College of Nursing's England director Mike Adams said the country was in the "eye of the storm" and the situation was "unsustainable".
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He told Sky News: "We did go into this pandemic with a huge shortage of healthcare staff, in particular nurses.
"If you then add in the exhaustion, the tiredness, the sickness rates that have begun to creep in, this is the outcome that no-one wants but it is really a last resort for people to come in off their leave.
"Long term, this will have a detrimental effect on workers, people need a break, they have got to get their rest.
"So, it is the eye of the storm and it is a situation that is unsustainable."
