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There are signs the UK is in the early stages of a third wave of coronavirus infections, a scientist advising the government has said.
Prof Ravi Gupta, from the University of Cambridge, said although new cases were "relatively low" the Indian variant had caused "exponential growth".
He said ending Covid restrictions in England on 21 June should be postponed.
Environment Secretary George Eustice said the government could not rule out a delay to the planned lockdown easing.
On Sunday, the UK reported more than 3,000 new Covid infections for a fifth successive day.
Prior to this, the UK had not surpassed that number since 12 April.
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Asked on BBC Radio 4's Today programme whether the UK was already in a third wave of infections, Prof Gupta said: "Yes, there has been exponential growth in the number of the new cases and at least three-quarters of them are the new (Indian) variant.
"Of course the numbers of cases are relatively low at the moment - all waves start with low numbers of cases that grumble in the background and then become explosive, so the key here is that what we are seeing here is the signs of an early wave."
However, he said the number of people who have been vaccinated in the UK meant this wave would probably take longer to emerge than previous ones.
"There may be a false sense of security for some time, and that's our concern."
Chart showing the number of UK infections is rising slowly
Prof Gupta - a member of the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) - said ending restrictions in June should be delayed "by a few weeks whilst we gather more intelligence".
"If you look at the costs and benefits of getting it wrong, I think it is heavily in favour of delay, so I think that's the key thing," he added.
Another leading scientific adviser to the government, Prof Adam Finn, also urged caution over the 21 June date for easing restrictions.
"I think it's unfortunate that everyone's got this particular date in their head, because really what we need to do is understand how things are going and adjust accordingly," he told ITV's Good Morning Britain.
"This time around, we should be cautious, wait to see what's happening, and then let everyone free, if you like, once we know for sure that that's safe and that we can do that without having another round of lockdowns."
The final stage of the government's roadmap for lifting lockdown, which would remove all limits on how many people you can meet - either indoors or outdoors, is due no earlier than 21 June.
However, Mr Eustice told the BBC the government had to take things "one step at a time".
"We can't rule anything out. We know this has been a difficult pandemic, a dynamic situation. We have to make that judgment a couple of weeks before.
"It will only be by then that we will see the impact of the latest easement we made on 17 May."
A final decision on whether restrictions will be lifted in England will be reached on 14 June.
The timetable for relaxing Covid measures varies across the UK. The Scottish government hopes to lift more restrictions on 7 June, while in Wales there is due to be a review on 3 June. In Northern Ireland some measures were relaxed on 24 May - the next review is due on 10 June.
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Analysis box by Nick Eardley, political correspondent
Three weeks today has been dubbed Freedom Day by some - the point at which the remaining restrictions are due to be lifted in England.
But there's increasing nervousness about whether it can go ahead in full because of the Indian variant.
And now you're seeing some of the different pressures the government is going to have to weigh up.
Nervtag scientist Prof Ravi Gupta is urging minister to postpone 21 June by a few weeks to give more time for second vaccination doses to be done.
In the Telegraph, the British Chambers of Commerce warns the government against "rolling back on the roadmap" - saying it would be "astonishing" for the government to change tack.
And ministers? They say it's too early to make a firm decision. They're waiting for more data over the coming fortnight. Many will be watching to see what it says.