Boris Johnson has said he has “serious concern” about the potential for rising Covid cases and hospitalisations to lead to more deaths.
But the prime minister said he has not yet made a decision on whether to delay the June 21 final lifting of restrictions.
Johnson stressed he would continue to study the data before announcing on Monday a decision on step four of the roadmap out of lockdown.
But he said he was less optimistic about the Covid situation than he was at the end of May.
It comes amid reports that the PM is mulling over either a two or four-week delay in the final lifting of restrictions.
In a series of interviews at the G7 summit in Cornwall, Johnson suggested he would not reimpose any restrictions, stressing that “the scientists have agreed” that there is no case for “going reverse”.
But commenting on rising cases of the Delta variant, which was first detected in India, 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.
“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 said: “Yes, that’s certainly fair.
“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.
“Some of the data is still open to question, but we’ll be making an announcement on Monday.”
Asked by Channel 5 News if he had already made a decision on June 21, the PM said: “No, we are going to be looking at more data.”