The prime minister said he has not yet made a decision on whether to delay the June 21 final lifting of restrictions but conceded there are “grounds for caution”.
Johnson said 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.”
But he acknowledged there are “grounds for caution”, telling BBC News: “We’ll be setting the position on Monday.
“But of course, you’re right, that you’ve got to be cautious in order to deliver what we want to see, which is an irreversible roadmap.
“But we’re looking at the data today.
“Again, you’ve got hospitalisations up, you’ve got, you’ve got cases up.
“There are grounds for caution.
“That doesn’t mean that this country hasn’t made enormous progress with vaccination.
“Clearly, what you’ve got is a is a race between the vaccines and the virus and the vaccines are going to win.
“It’s just a question of pace.”
Johnson also defended his decision to wait until April 23 to put India on the red travel list, despite a huge surge in cases in the country in spring.
He has faced criticism over the decision to wait two weeks after putting neighbouring Pakistan and Bangladesh on the red list to add India.
The PM told ITV News: “We put India on the red list so that you have to spend 10 days quarantining in a hotel if you come from India, which was before the variant was identified as a... a very deep concern.”
Asked if he regretted waiting on the red list decision, Johnson said: “Obviously we’ll look back at all the decisions we made but that was done on the best scientific advice at the time, and the situation in Pakistan and other countries was very different.
“As it happens, what you’re looking at with the Indian variant is, it’s one that I’m afraid is now very widely ceded around the world.
“But it is susceptible to the vaccine so it’s susceptible to two doses, so great thing is for everyone to get their second dose, that’s what we need.”
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.