Boris Johnson has said the spike of coronavirus cases is of "serious, serious concern" as he prepares to put lockdown lifting on hold.

The Prime Minister has cast fresh doubt on lifting the remaining lockdown restrictions on June 21 as he expressed concerns about the surge in cases of the Delta variant.

The total number of infections attributed to the mutation, which was first detected in India, now accounts for more than 90% of new Covid cases across the country.

The Prime Minister is now said to be considering whether to order a two-week or four-week postponement to the lockdown end date that was pencilled in for June 21, the Mirror Online reports.

In a series of interviews at the G7 summit in Cornwall, he fuelled fears that the date for ending lockdown may be delayed.

He told Sky News: "It's clear that the Indian variant is more transmissible and it's also true that the cases are going up, and that the levels of hospitalisation are going up.

“We don't know exactly to what extent that is going to feed through into extra mortality, but clearly it's a matter of serious, serious concern."

Latest figures show the Delta strain is about 60% more infectious than the Kent variant which drove the winter surge and second wave.

Another 8,125 cases were diagnosed in the last 24 hours, with the rolling seven-day average up 58%.

Asked to guarantee he would not go backwards on the roadmap, he said: "The situation is dramatically different from what it was in either of the first two waves of the pandemic.

“Yes, it is absolutely correct to say that you've got cases going on up now from the Delta variant and you've got an increase in hospitalisations.

“But you're seeing a different group now going into hospitals, largely and the outcomes, on the whole, are better, but that's caused by the huge effectiveness of the vaccine rollout.”

Experts are not advising that restrictions are tightened, the PM said.

He added: “Just to cheer you up a little bit, I would like to tell you that the scientists are agreed about one thing – they do not think there is any case for going reverse.”

Public Health England now believes the Delta variant accounts for more than 90% of new Covid cases across the country.

The total number of infections attributed to the mutation now stands at 42,323.

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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 told BBC Radio 4’s Today programme on Saturday: “This Delta variant seems to be about 60 per cent more transmissible than that (the Alpha, or Kent, 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.