Dr Hilary Jones has warned the public won't be able to see the effect of reopening schools, hair salons, pubs, bars and restaurants until up to four weeks' time.
The TV doctor, 67, addressed that coronavirus infection rates were still going down and was asked by GMB host Ranvir Singh whether we could trust the easing of lockdown yet.
He explained on Good Morning Britain: "Well, we won't know about the effects of easing lockdown for another two to four weeks.
"So we need to do the test and tracing to find out what's going on with the prevalence rates, what's going on with positive cases and indeed what's going on in hospitals.
"So there will be a time lag, if you like. We haven't got enough data to know what the affect of opening schools has been.
"It's really just a case of 'watch this space' and being very cautious and acting early before we see a spike in the number of cases."
Presenter Ben Shephard probed Dr Hilary on whether we would have noticed and immediate reaction following the opening of pubs last weekend.
"An surge now would have been the result of what was happening two or three weeks ago," Dr Hilary replied.
"Because we know about the incubation period of the virus, we know how long it takes for symptoms to start and that's why self-isolation has always been 14 days.
"One of the problems is that there is always this lag and we have to act quickly when we see a little spike because we know that's the beginning of an upward trend. And that's why the r-rate is so important and we keep tabs on that."
Yesterday, Culture Secretary Oliver Dowden announced that beauticians, tattooists and tanning salons will be able to reopen from Monday.
But facial treatments, such as face waxing, make up application and eyelash treatments won't be allowed because of the "much greater risk of transmission".
Screens and barriers should be in place to separate clients from each other.
*Good Morning Britain airs weekdays at 6am on ITV
