It comes after Boris Johnson said on Monday there is a “good chance” the one-metre plus rule for social distancing can be ditched, in England at least, next month.
The final decision on whether the change can be brought in from June 21 will depend on the data, the Prime Minister added.
Professor Stephen Reicher, from the University of St Andrews and a member of the Sage sub-committee, told BBC Breakfast: “I think we should take this with a little bit of a pinch of salt.
"Remember, he said it in the middle of an electioneering visit to the North of England, and clearly he wants to tell a good news story… He immediately qualified it by saying it depends on the data and how many infections there are and the state of things on June 21, nearly two months away.
“Now, if a week is a long time in politics, two months is an eternity in a pandemic. Remember, two months ago in India they were declaring the pandemic was all over, now they’re having 400,000 cases a day.
“So things can change very rapidly, but I think the critical issue for us now is how do we bring infections down so the data does look good, and we can relax things in two months’ time, and the real important issue is this – if we take this as a signal that things are all over, if we relax and if we mix now, the paradox is we will push up the infections and will make it less likely that we can relax on June 21.”
He said that “if we believe there is no risk at all, if we start mixing without restraint, then we’re going to be in real trouble.
“And, as the World Health Organisation put it very clearly, one of the biggest things is complacency, so I think we need to be optimistic, but vigilant as well.”