BORIS Johnson has vowed not to delay the easing of lockdown even longer and is confident the country will unlock fully in four weeks.
The Prime Minister pushed back the June 21 Freedom Day plans this evening and confirmed step four of his roadmap will now take place on July 19.
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Mr Johnson added he is "confident" no further delay will be necessary - unless the data worsens or a new variant emerges.
The PM told the nation: "I am confident we will not need more than 4 weeks and we will not go beyond July 19.
"But now is the time to ease off the accelerator."
Prof Chris Whitty and Sir Patrick Vallance came armed with alarming data that the Delta variant - which was first identified in India - was up to 80 per cent more infectious and was doubling in a third of places.
NO DELAY
But the experts said the jabs are "highly effective" against the Delta variant.
Hospitalisation rates are increasing by an average 15 per cent every week - and a staggering 66 per cent in the North West.
The PM warned that removing all legal restrictions on social contact on June 21 - as previously planned - would see the NHS inundated with Covid patients.
Mr Johnson said that by July 19 there will be a "very considerable wall of immunity" in the population - but he also warned that it is impossible to definitely rule out a potential new variant which does not exist at the moment.
He said: "At that stage, on the basis of the evidence that I can see now, I'm confident that we will be able to go forward with the full Step 4, the full opening."
But he added: "That, of course, does not exclude the possibility - I'm afraid, and we have got to be honest about this - the possibility that there is some new variant that is far more dangerous, that kills people in a way that we currently cannot foresee or understand."
He explained: "Looking at the data now, looking at the effectiveness of the vaccines, I do think we'll be able to move to Step 4 on July 19.
"At that point people may want to keep social distancing, and keep being sensible, but I think we'll be in far better position as a country."
Mr Johnson added: "I think it is sensible to wait just a little longer. We will hold off until July 19.
"I am confident we won't need more than four weeks - it's unmistakable clear that the vaccines are working - but now is the time to ease off the accelerator.
"We have the chance to save many thousands of lives by vaccinating millions more people."
During the press conference, the PM also confirmed that people aged 23 and 24 will be able to book their Covid-19 vaccines from tomorrow.
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He said all adults will be offered a vaccine by July 19.
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It comes as a new PHE study found that unvaccinated people have twice the risk of hospital admission with the Delta variant as the Alpha variant.
Mr Johnson said that a delay could give the NHS "a few more crucial weeks to get those remaining jabs into the arms of those who need them".