Ministers are considering putting back the relaxing of restrictions planned for June 21 for up to four weeks as they race to roll out the vaccine to younger age groups.

Boris Johnson looks set to delay the final lifting of restrictions in England following another sharp rise in cases of the Delta variant. A final decision is expected to be taken on Sunday ahead of a formal announcement by the Prime Minister at a news conference the following day.

Experts have warned, the Government should be “cautious” about fully lifting lockdown measures in England due to the rise in cases of the Delta variant which could lead to hospitals being overwhelmedT

Professor Peter Openshaw, a member of the Nervtag advisory group, said that it was a “disappointing setback” that the Delta variant first identified India seemed even more successful than the previous strains.

He said: “This Delta variant seems to be about 60% more transmissible than that (the Alpha variant). So it really has gone up another gear and that means that we really have to double down and not lose all the advantage that has been gained by the massive effort that has been put in so far.”

Professor Tom Solomon, director of the Health Protection Research Unit in Emerging and Zoonotic Infections at the University of Liverpool, said the country could not afford a “bad decision” on unlocking.

He said that while vaccines were having a “massive impact”, opening up could lead to hospitals being overwhelmed.

Prof Solomon added: “If you look at hospitalisations, they are doubling – the numbers are small but they are doubling approximately every seven days – and so if you then suddenly say we are going to open up completely we may end up with the hospitals overwhelmed again.

“So I think, unfortunately, we are just going to have to maybe give it another month until we have so many more people vaccinated.”

Foreign Office minister James Cleverly said it was “key” that the country did not trip up at the final hurdle and that restrictions will be lifted in “a way that is safe”.