Second coronavirus vaccine doses will be brought forward by a month to tackle the Indian variant after Government scientists said the mutated virus may be 50 per cent more transmissible.
Boris Johnson announced on Friday that booster shots will be accelerated across the country for the over-50s and clinically vulnerable, with the gap between jabs slashed from 12 to eight weeks.
Under new plans, doses will also be prioritised for the over-40s who have not yet come forward, amid fears that vaccine hesitancy is driving the surge in cases in areas like Bolton and Blackburn, which have seen a far lower take-up than elsewhere.
However, the Prime Minister stopped short of bringing forward vaccines for all over-18s, despite widespread calls to target young people who appear to be spreading the variant.
New coronavirus cases involving the variant known as B1.617.2 have more than doubled in a week in England, with London and the North West seeing the biggest rise in infections.
Papers released last night from the Scientific Pandemic Influenza Group on Modelling (SPI-M) which feeds into Sage, suggest the Indian variant may be 50 per cent more transmissible than the Kent variant, which was already around 20-30 per cent more infectious than the original strain.
Although 36 million Britons have had at least one dose of their jab and 19 million are now fully vaccinated, SPI-M warned it was still too few to be safe.
SPI-M warned: "Considering this, it is a realistic possibility that this scale of B.1.617.2 growth could lead to a very large increase in transmission.
"At this point in the vaccine roll out, there are still too few adults vaccinated to prevent a significant resurgence that ultimately could put unsustainable pressure on the NHS, without non-pharmaceutical interventions."
Latest Sage minutes warn that if the variant had a 40-50 per cent transmission advantage, continuing with Monday’s release "would lead to a substantial resurgence of hospitalisations similar to, or larger than, previous peaks".
Government modellers said the variant could lead to 1,000 Covid deaths a day by summer, and 10,000 daily hospital admissions.
Speaking at a televised Downing Street press conference, the Prime Minister said the country was now involved in a tighter race between the vaccination programme and the virus, but confirmed Monday’s lifting would still go ahead.
He said he was hopeful that vaccines would prevent the Indian variant from causing deaths and admissions, even if mild infections rose.
"We’re in a very different world now," he said. "If the variant is significantly more transmissible we’re likely to face some hard choices.
"The good news is that we have no evidence our vaccines will be less effective in protecting people against severe illness and hospitalisation. So we are in a different position from the last time we faced a new variant before Christmas."
Scientists were forced to weigh up the benefit of getting those most at risk from the virus fully vaccinated sooner, with the higher level of effectiveness thought to be gained by delaying second doses by up to 12 weeks.
A paper out this week from the University of Birmingham showed people who wait 12 weeks for a jab have significantly more antibodies than those given their booster after three weeks.
While the Government decided to change its advice on the gap between doses, it rejected calls from Greater Manchester Mayor Andy Burnham and Bedford Borough Council to vaccinate young workers and students.
Latest data suggests the rise in cases is predominantly in young, unvaccinated people, yet experts said they were unlikely to suffer severe disease and it was better to save jabs for older people.
Professor Chris Whitty, the Chief Medical Officer for England, said it was possible vaccines had already created a "firebreak" for older people, and that a rise in infections in the young would not lead to a rise in admissions and deaths.
"The thing we know is this vaccine can and should do is to protect the most vulnerable and that is very much predicated by age," he said.
But he warned that the Indian variant would likely become the dominant strain in Britain.
More vaccine doses have been sent to Bolton, while 800,000 PCR tests have been sent to 15 separate areas of England, including parts of London and Merseyside.
London and the North West have seen the biggest rise in cases of the variant, with Public Health England data showing it has been responsible for four deaths as of May 12.