Professor Adam Finn from the University of Bristol, also a member of the Joint Committee on Vaccination and Immunisation (JCVI), said on Monday that "things are much more up in the air" than they were earlier this month due to the spread of the B.1.6172.

"In a way, I think, the dates (of the roadmap) all the way through were created just to give everyone some kind of sense of structure," Xinhua news agency quoted Finn as saying while speaking to Sky News.

"But they were always qualified as being based on what actually happened and that they could change. I think there's a significant chance that it could change," he said.

"We're better off being cautious at this point and being able to progressively unlock ourselves than to overdo it and then end up having to lockdown fully all over again."

His warning came as Britons gathered on beaches, in parks and beer gardens to enjoy the bank holiday on Monday.

Cases of the B.1.6172 variant have doubled in a week in England to almost 7,000, prompting concerns that the government's lockdown roadmap will be derailed.

The roadmap is expected to see all legal limits on social contact to be removed on June 21.

It is understood that a final decision on the planned easing of lockdown will not be made until June 14.