Boris Johnson has admitted he is less optimistic about the easing of lockdown restrictions than he was last month and offered a strong hint he will delay the 21 June reopening.

The prime minister, speaking to Sky News at the G7 summit in Cornwall, said he was prepared to be “cautious” over the fourth and final stage in his roadmap for lifting COVID measures.

Although Mr Johnson previously hoped to remove all remaining legal restrictions on social contact on 21 June, it has been suggested he could now delay step four of his roadmap by as much as a month.

The prime minister said it was “clear” the Delta variant of COVID, first discovered in India, was “more transmissible” with coronavirus cases and the number of people in hospital increasing in the UK.

“Now we don’t know to what extent that exactly is going to feed through into extra mortality but, clearly it’s a matter of serious, serious concern,” he told Sky News political editor Beth Rigby at the G7 summit venue in Carbis Bay.

Mr Johnson acknowledged it was “certainly fair” to say he was less optimistic than he was at the end of May.

And – signalling he could announce a delay to stage four when he delivers a COVID update on Monday – the prime minister added: “What we want to do is make sure that the roadmap is irreversible – but you can’t have an irreversible roadmap unless you’re prepared to be cautious.”

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However, Mr Johnson offered some confidence that he would not need to reintroduce previous COVID restrictions to deal with the impact of the Delta variant.

“Just to cheer you up a little bit more, what I can tell you is the scientists are agreed about one thing – they do not think there is any case for going into reverse,” he said.

“What we’re looking at is, yes absolutely, an increase in cases, we’re seeing an increase in hospitalisations.

“But the context has radically changed because of the sheer number of people who have been vaccinated and particularly the elderly and vulnerable.

“So the objective now is to give that vaccination programme the legs, the impetus, the speed it needs to beat the spread of the virus. And I’ve got no doubt that we can.”

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Mr Johnson said the situation at present was “dramatically different from what it was in either of the first two waves of the pandemic”.

“Yes, it is absolutely correct to say that we’ve got cases going up now from the Delta variant and you’ve got an increase in hospitalisations,” he added.

“But you’re seeing a different group now going into hospital, largely, and the outcomes on the whole are better.

“That is caused by the huge effectiveness of the vaccine rollout.”