The government is due to make a decision on whether to remove all limits on social contact on 21 June a week before on 14 June.
Cases in England soar by 76.5% in past week, according to latest ONS figures.
Daily reported test results also suggest a sharp rise with 6,278 confirmed cases on Friday.
Epidemiologist Dr Adam Kucharski has said he is “nervous” about the idea of unlocking later this month amid the spread of the Delta (Indian) variant.
“I think we have to accept the equation has changed here – we are not facing the virus that we were facing two months ago,” he told BBC Radio 4’s Today programme.
“If we were facing the B.117 (Kent) variant as the dominant one, the fact it (the vaccines) is working very well, case numbers are coming down nicely, we could have some more confidence that there could be reopening without seeing those surges.
“But we’re not. Everyone is tired of this pandemic, they want this transition back to normality and that will happen, the effectiveness of two doses of the vaccine will eventually get us there.
“But if say in two weeks we’re in a situation where hospitalisations have been rising, where local health systems are coming under pressure, I think it will be quite difficult to justify adding more transmission to that kind of situation.”
Dr Adam Kucharski, TED
The chief executive of NHS Providers says the Delta variant is “going to become the dominant variant” and is now “starting to spread across the country”.
In those areas, what the NHS is trying to do is surge vaccinate, says Chris Hopson. It will spread “even faster” if all restrictions on social contact are removed in England on 21 June, he adds.
Asked if the easing should go ahead as planned then, he says that “in the previous lockdowns, we have had to do them because there has been no choice – we had to do them to ensure the NHS wasn’t overwhelmed”.
This time the decision is “significantly more finely balanced” – because “we know we hopefully have broken the chain” but there are risks associated with the Delta variant too, as it is more transmissible, and also because many people are still unvaccinated and the NHS is busy.
Instead of 21 June being an “all or nothing” day, he says he suspects some things will be able to happen from that day – but others will be “too high risk” still.