BORIS Johnson has warned the spread of the Delta variant is of "serious, serious concern" as he prepares to put the breaks on lifting lockdown in England.

The Prime Minister is expected to announce a delay of up to four weeks in the final easing of restrictions, which had been due to take place on June 21 under the UK Government's road map.

Speaking during the G7 summit in Cornwall, he insisted that no decisions had been taken ahead of a formal announcement on Monday.

However he made clear that there had been a deterioration in the situation, with a surge in cases of the Delta variant – first detected in India – since the start of the month.

The Tory leader, who was repeatedly urged last month to introduce stricter travel measures to prevent the importation of the variant, said: "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.

"Now, 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."

Asked if he was less optimistic now than he was at the end of May, he told Sky News "Yes, that's certainly fair.

"What we want to do is make sure that the road map is irreversible, but you can't have an irreversible road map unless you're prepared to be cautious.

"Some of the data is still open to question, but we'll be making an announcement on Monday."

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His comments come as experts urged the UK Government to be "cautious" about fully lifting lockdown measures in England due to the rise in cases of the Delta variant.

Professor Peter Openshaw, a member of the Nervtag advisory group, said that it was a "disappointing setback" that the variant seemed even more successful than the previous strains.

He told BBC Radio 4's Today programme: "This Delta variant seems to be about 60% more transmissible than [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 UK could not afford a "bad decision" on unlocking.

He told BBC Breakfast that while vaccines were having a "massive impact", opening up could lead to hospitals being overwhelmed.

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."

Tory ministers are considering putting back the relaxing of controls for up to four weeks as they race to roll out the vaccine to younger age groups.

Johnson told ITV News: "We're looking at all the data but what we're wanting to do is avoid another wave of deaths that could be prevented by allowing the vaccines to work in the way that they are.

"The vaccine programme has been absolutely astonishing and there's no question that if you look at the people going into hospital now they tend to be in different groups, younger groups, than we saw in the first couple of waves of the pandemic.

"But it may be that in the race between the vaccines and the virus, we need to make sure we give the vaccines extra legs."

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.

Scientists now estimate that 96% of all new cases of coronavirus are attributed to the Delta variant.

The latest figures from Public Health England (PHE) showed there have been 42,323 cases of the Delta variant confirmed in the UK, up by 29,892 from the previous week.

It estimates the strain is 60% more transmissible compared with the previously dominant Alpha, or Kent, variant, and that cases are doubling every four-and-a-half days in some parts of England.