Dr Hilary Jones urged caution over a change to self-isolation rules in England, following the government's decision.
Speaking on ITV's Good Morning Britain on Monday, when the new rules came into place, Dr Hilary warned two negative tests did not guarantee someone was no longer infectious.
He urged people to take precautions if they return to society after five full days, even if they have tested negative for the virus.
Hilary added that easing all restrictions too soon "would be a mistake", amid reports that rules could change next week.
Previously people who tested positive for Covid could be freed midway through their seventh full day of isolation, if they tested negative on days six and seven.
Now, under the government's new rules, people will have to isolate for at least five full days, starting at 12.01am the day after their positive test or the start of symptoms.
They will then be able to leave isolation at the start of the sixth full day, if they have tested negative for Covid on both day five and day six.
Speaking on ITV's GMB on Monday, he told hosts Richard Madeley and Susanna Reid that negative lateral flow tests on day five might not guarantee someone is no longer infectious.
Richard had said: "I don't know anybody who tested negative on day five or six. I tested negative on day 10 or 11, and most people I know were well over a week."
Hilary responded: "We know that 30 per cent of people are still infectious by day five, but if you have two negative tests it's thought according to the data that your infection rate is about seven per cent.
"So those two negative lateral flow tests, which you have to report on the gov.uk website if you're going back to work or education, you have to report that, otherwise it's a default period still of 10 days.
"But two negative tests on day five and six mean you can return to work, but they do say to still wear a mask, and avoid close contact with people, especially in unventilated areas."
Hilary went on: "What we have to remember is a negative test is great. A positive test means you've got viral particles still there which the test will detect, it doesn't necessarily mean you're infectious.
"It's detecting virus that is no longer viable but it doesn't really tell you whether you are infectious or not, it's an indication."
Susanna then asked: "If you test negative on day five or day six, are you definitely not infectious?"
Hilary told her: "No, you could be infectious. As I say 30 per cent of people on day five are infectious and could have negative tests, it's not a guarantee that you are not infectious, which is why they are saying to still wear masks and take extra precautions."
He then added how booster jabs being approved for younger age groups is "another step forward", as he addressed reports of the lifting of restrictions being looked into.
Hilary added: "They really want to pretend the pandemic is over and that we are out of it, and we're living an endemic virus now, and everyone can carry on as normal.
"We have to be careful, the R rate still is 1.1-1.5 which means 10 people can give the virus to 10, 11 other people, up to 15 I think.
"It's still growing. The rate for Omicron is slowing down and hopefully we will reach a stage where we have got herd immunity in the majority of the population, and we can start easing restrictions but if we do it all at once, I think it would be a mistake."
Good Morning Britain airs weekdays at 6am on ITV.
