THE official R rate in England could be as high as 1.3 in the North West - a hotspot hit hard by the Indian variant.
It comes after experts warned the mutation could be 100 per cent more infectious than the Kent variant, which caused the country to lockdown in January.
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Variant fears mean June 21's "Freedom Day" could be delayed, if cases keep going up and the mutations cause havoc.
Professor Neil Ferguson, from Imperial College London, whose modelling was instrumental to the UK locking down in March 2020 said vaccines are helping, but warned the Indian variant is spreading.
Speaking to BBC radio 4's Today Programme he said: "There's some uncertainty around that depending on assumption and how you analyse the data, between about 30 per cent and maybe even up to 100 per cent more transmissible."
Whether or not the next stage of the roadmap will go ahead is currently is not decided, but there is mounting pressure from scientists to hold back.
But this week Boris Johnson said he would still today still pressing on with plans to end lockdown this month - despite warning the decisive data was still "ambiguous".
It comes as official figures showed Covid cases in England have soard 75 per cent in seven days to 86,000 last week.
The UK is at the “start of a Covid epidemic in the young”, according to data, as hotspots threaten the lockdown lifting on June 21.
R-ISING RATE
The R rate in England is now 1.0 to 1.2. according to Government advisers at Sage. Last week Sage said the value was 1.0 to 1.1.
An R rate below 1.0 is favoured because it means that the outbreak is still shrinking.
It peaked on January 15 at between 1.2 and 1.3, came down to a low of 0.6 to 0.8 in mid-March, before creeping up again over the past few weeks since Brits were given more freedoms.
Prof James Naismith, Director of the Rosalind Franklin Institute, and University of Oxford, said: “There are two factors at play here, once is the easing of the lockdown measures in May and the second is the delta variant (which has now become dominant).
“The trajectory of the case numbers in the North West are particularly worrying. East Midlands appears on a similar track.
"We are seeing some evidence for an increase in hospitalisation where the case numbers are highest. Without vaccines, we would seem to be a the start of a third wave and given the nature of the delta, such a third wave could have been particularly disastrous.
“It is worth pausing to remember that if it is as transmissible and severe as early data indicate the delta variant will devastate less developed countries. I am filled with dread and sorrow for what lies ahead."
There are fears the Indian variant and a mutated version of the strain, dubbed the "Nepal variant" will force "Freedom Day" to be delayed, after cases doubled in a week.
But teens aged 12 to 15 will be able to sign up to get a Pfizer vaccine when their slot comes to get jabbed - after approval was confirmed today.
Dr June Raine, MHRA Chief Executive said: “We have carefully reviewed clinical trial data in children aged 12 to 15 years and have concluded that the Pfizer/BioNTech COVID-19 vaccine is safe and effective in this age group and that the benefits of this vaccine outweigh any risk.
“We have in place a comprehensive safety surveillance strategy for monitoring the safety of all UK-approved COVID-19 vaccines and this surveillance will include the 12- to 15-year age group."
It comes as the Office for National Statistics published new data showing the Indian (Delta) and Kent (Alpha) variants have seen an increase in cases in hotspots.
The Indian variant has risen in England, while cases of the Kent variant have increased in Wales.
The ONS estimates 85,600 people, or 0.16 per cent, in England had Covid in the week running up to May 29.
Areas in the North West, East Midlands, and South West saw a rise in infections, with the West Midlands and London starting to see a possible increase.
Adding to the fears the UK is seeing the pandemic hit the youth now, the new data also showed an increase in positive cases in Year 7 to Year 11 schoolkids.
Sarah Crofts, Head of Analytical Outputs for the COVID-19 Infection Survey, said: “Our figures today show a mixed picture across the UK.
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"Although infections remain low, the percentage of those testing positive for COVID-19 has increased slightly in England and we are seeing early signs of an increase in Wales. The trends are uncertain in Northern Ireland and Scotland.
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“We are also seeing an increase in England in the percentage of cases compatible with the Delta variant first identified in India.
“Analysing these trends will be key over the next few weeks as we monitor the impact of the variants in different regions.”