Boris Johnson has warned that Covid hospitalisations are rising, as the scientist behind the first lockdown said admissions could hit the same heights as last winter.
The Prime Minister on Wednesday suggested it was too early to say whether restrictions can be lifted on June 21, as data on whether vaccines offer enough protection against the Indian or "delta" variant are still being assessed.
It came as the Government reported 7,540 new cases on Wednesday – the highest single-day rise since Feb 26.
"On Monday… we’ll have a look at where we are. I think what everybody can see very clearly is that cases are going up, and in some cases hospitalisations are going up," Mr Johnson said.
"What we need to assess is the extent to which the vaccine rollout, which has been phenomenal, has built up protection in the population in order for us to go ahead to the next stage. And so that’s what we’ll be looking at."
Prof Neil Ferguson, of Imperial College London, warned that the most recent modelling showed a "risk of a substantial third wave".
However, he said that while this could see a surge in people admitted to hospital, it was unlikely to trigger a comparable rise in deaths.
There is a lag of up to two to three weeks between infections and hospitalisations and deaths, and scientists are waiting to see the effect of the present increase in cases, driven by the Indian variant.
The most crucial data will be that which indicates the extent to which the vaccines protect people against hospitalisation and death.
Prof Ferguson said that "in the next few weeks" experts hope to be able to see more clearly what the ratio is between hospital admissions and cases, though it was already known that "vaccination has fundamentally changed that ratio" for the better.
The Government has promised to update the country on the proposed June 21 reopening next Monday. Asked whether delaying the date would make a difference, Prof Ferguson said: "Yes, because it allows more people to get second doses."
He said efficacy for the second dose against the Indian variant was higher than after one, adding that a delay would also protect people and "have an effect on transmission, of getting more weeks of getting people vaccinated".
Prof Ferguson said any third wave "will translate, by definition, into some number of hospitalisations and deaths" but added that it is "harder to pin down quite how significant the latter will be at the current time".
He said it was currently difficult to resolve how cases "will translate into hospitalisations, but it’s well within the possibility that we could see a third wave at least comparable in terms of hospitalisations, maybe not as severe, as the second wave".
"Almost certainly I think deaths probably will be lower – the vaccines are having a highly protective effect, cases in hospital now are milder – but it still could be quite worrying," he added.
How many people have been vaccinated?
On Wednesday, Chris Hopson, the chief executive of NHS Providers, said hospitals in Covid hotspots are seeing a "significantly" lower death rate among people admitted for treatment and are coping with current levels of infection.
He said there was a degree of confidence that vaccines have "broken" the link between infections and the "very high level of hospitalisations and mortality we’ve seen in previous waves".
He told Times Radio: "And if – and it is a big if – if Bolton has gone through its complete cycle and if other areas follow Bolton, the view from the hospital there was that they were able to cope with the level of infections.
"It’s important not to just focus on the raw numbers here – you also do need to look at who’s being admitted into hospital and how clinically vulnerable and what level of acuity they’ve got.
"What chief executives are consistently telling us is that it is a much younger population that is coming in, they are less clinically vulnerable, they are less in need of critical care and therefore they’re seeing what they believe is a significantly lower mortality rate, which is, you know, borne out by the figures."