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Boris Johnson says it is clear that Covid cases are rising and hospitalisations increasing, as a decision on 21 June unlocking looms.
Official figures show more than 1,000 people in the UK are currently in hospital with the disease.
The PM said he needed to assess the extent to which the vaccine rollout has built up protection in the population before progressing with the roadmap.
And he said again that the decision for England would be "driven by the data".
The number of people in hospital with the virus has risen above 1,000 for the first time since mid-May.
Scientists believe this could be the start of the third wave, caused by the Delta variant.
The size of that wave depends on how effective vaccines are at protecting people - and that is still uncertain.
The government is due to decide on 14 June whether to lift the last remaining restrictions in England.
But government sources are more downbeat about the prospect of the easing going ahead the following week.
Speaking in Cornwall ahead of the G7 summit, the prime minister said there are "arguments being made one way or another" as to whether all restrictions can be lifted as planned.
"What everybody can see very clearly is that cases are going up and, in some places, hospitalisations are going up.
"What we need to assess is the extent to which the vaccine rollout, which has been phenomenal, has built up enough protection in the population in order for us to go ahead to the next stage."
Mr Johnson added: "The reason we've been doing the steps on the roadmap with five-week intervals is to give us time to look at all the data as it comes in and access the state of the pandemic before we go forward to the next step.
"So on Monday that four-week period will be up - and we'll look at where we are."
It comes as one million under 30s booked vaccine appointments in England on Tuesday - and data showed more than half the adult population is fully vaccinated.
Prof Neil Ferguson, virus modeller from Imperial College London whose predictions were key to the first lockdown in March 2020, has said that delaying unlocking would allow more people to be vaccinated.
While a first dose gives some protection against the Delta variant, first detected in India, the second dose appears to give much more.
Another 7,540 people tested positive for the coronavirus in the UK on Wednesday - the highest daily number of cases since the end of February, the latest government figures show.
There were also a further six deaths reported within 28 days of a positive test. A week ago there were 4,330 cases and 12 deaths.
A total of 1,024 people are now being treated in hospital in the UK, up 69 on the previous figure.