If the Indian variant continues to rise over the next seven days, then the 21 June date for lockdown to end could be extended.
Experts are warning that hospital admissions are slowly starting to rise and are up by a 11% week on week rise and daily positive results rose on Wednesday by over 3,000 cases, the first time since 12 April.
Professor Neil Ferguson who pursuaded Boris Johnson to initiate the first lockdown, warned scientists are still uncertain as to how much more transmissible the Indian strain is in the UK.
Professor Ferguson said it could be up to a further three weeks before they know for certain if lockdown can end on 21 June.
If the vaccines are effective, and the Indian variant is only up to 30% more transmissible, then a third wave could be far smaller.
Speaking to an event organised by the Science Media Centre in Germany on Wednesday he said, “It’s not how high cases rise, but how quickly.
“If they double every 10-14 days and hospital admissions follow the same trend, then there is a concern.
“We were expecting cases to rise as we relaxed restrictions but if they rise too quickly that’s a problem.”
Professor Ferguson told the BBC’s Today programme the 21 June date now “hangs in the balance.”
He added, “In the next two or three weeks we will be able to come to a firm assessment of whether it’s possible to go forward.”