The coronavirus variant first discovered in India could potentially be up to 100% more transmissible than the Kent version, a leading epidemiologist has warned.
Professor Neil Ferguson said that early data around the variant - which has been renamed 'Delta' by the World Health Organization - has placed the increased transmissibility anywhere between 30% and 100%, thought the current best estimate is 60%.
Prof Ferguson, who is a member of the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said the signs were "pointing in a more negative direction" and that caution is needed around any easing of restrictions across the UK in the coming days.
His comments come as initial data from Public Health England (PHE) suggested that the Delta variant is more likely to lead to hospitalisations.
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But Prof Ferguson stressed that most people being hospitalised at the moment with the Delta variant as well as other variants of the virus are unvaccinated.
A PHE report, which analysed 38,805 cases in England, suggested that the Indian/Delta variant was associated with 2.61 times higher risk of hospitalisation than the Kent/Alpha variant, with a 1.67 times higher risk of a patient needing care in A&E.
Findings from PHE’s 3 June risk assessment, determined with a “high degree of confidence”, also suggested that early data shows vaccines are less effective against the strain compared to the Alpha strain.
The PHE report came as research published in the Lancet by scientists at the Francis Crick Institute and the National Institute for Health Research added to evidence that COVID jabs are somewhat less effective against the Delta/Indian than the Alpha/Kent variant, particularly after the first dose.
Prof Ferguson told the Today programme: "We're certainly getting more data and unfortunately the news is not as positive as I would like on any respect about the Delta variant.
"The best estimate at the moment is this variant is certainly substantially more transmissible."
He added that more data is needed on how much the new variant can evade the immunity which protects people against hospitalisation.
Asked if the latest data could affect the further unlocking of the country on 21 June, Prof Ferguson said: "I think the data is pointing this week in a more negative direction than it was last week so it points towards the direction of being cautious.
"I think balancing people's desire - and there clearly is a built-up desire to get back to normal - against the potential risk is a very difficult judgement call."
He said at the moment the Delta variant is doubling across the country around every nine days, but said it still remained to be seen what the full extent of the latest stage of lifting restrictions on 17 May had had.
'This is bad news'
Prof Ferguson's comments come amid warnings from some experts about the rapid spread of the Delta variant.
Professor Christine Pagel, professor of operational research at University College London and a member of Independent Sage, wrote on Twitter: "I am getting sick of the constant attempts to explain away any bad news.
"I get the desire to be optimistic (honestly) but minimising delays action and ultimately makes things worse. Exponential growth is a red flag. Delta has been growing exponentially since early April.
"We *might* have been able to limit its spread then but it was "no firm evidence" & it kept growing. Instead we *now* have evidence that delta is (a lot) more transmissible, partially vax resistant & more severe. It's now 80% of our cases. The genie is out of the bottle.
"Any delays to the road map or a bad new surge are entirely on the govt. They should be the target of our anger. Not me, or any of other people who have been warning about this for weeks - which btw includes SAGE several times."
Deepti Gurdasani, senior lecturer in Epidemiology at Queen Mary University of London, wrote: "I can't begin to describe how angry I feel that a highly transmissible, more severe variant, with significant escape from vaccines was not only allowed to enter the country, but allowed to spread while our govt removed mitigations & minimised the risks posed by this."
She added: "Worryingly, it looks like the risk for hospitalisations both in Scotland and England data increases by ~2.5x for delta compared to alpha (Kent variant), confirming what many of us have suspected- that it's not just more transmissible, but also more severe."
On Thursday Susan Michie, Sage scientist and professor of health psychology, told a Royal Society of Medicine webinar that the UK is still at risk from a third wave, despite more than half of adults being fully vaccinated.
She said there were "two reasons" why the country should be concerned, despite the progress of the vaccine rollout, saying: "One is the higher transmission, the more mutations and therefore the higher the likelihood of a variant that could undermine vaccination, and secondly the issue of long COVID, which is very debilitating."
Their concerns come Scotland's Deputy First Minister and COVID Recovery Secretary John Swinney revealed that a 'high' number of children in Scotland are in hospital with COVID amid concerns that the Delta variant may affect younger people more.
Swinney told the BBC’s Good Morning Scotland: "The current numbers are on the high side, certainly over the period of COVID we’ve not seen very many children hospitalised but we’re seeing a number just now.
"So we have to look at all of these factors to determine is there something in the new variants that are emerging that is making it more acutely challenging for children with a greater health impact, and these are the issues that we keep under constant review and upon which we take clinical advice."
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