Dr Hilary Jones has urged people to be cautious after a major amendment to the coronavirus regulations came into effect today.
The medical expert was interviewed by Susanna Reid and Richard Madeley on Monday's edition of Good Morning Britain.
The broadcasters explored the new self-isolation rules as people who test positive for Covid-19 can now leave quarantine after five full days.
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People must be able to display a negative lateral flow test on days five and six to reduce the isolation period, but Dr Hilary warned this does not necessarily mean they are not infectious.
He said: "You could be infectious. As I say, 30% of people on day five are infectious and could have a negative test.
"It's not a guarantee that you are not infectious, which is why they are saying you should still wear a mask, take extra precautions and particularly avoid people who are immunosuppressed or are more at risk."
However, the doctor explained that anybody who receives two negative tests is likely to have a low infection rate.
He continued: "If you have two negative tests it's thought according to the data that your infection rate is about seven per cent.
"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."
The government has made the change to the rules in the hopes of reducing staffing pressures as positive cases have surged across the UK due to the spread of the Omicron variant.
Health Secretary Sajid Javid said: "Following a robust review of the evidence, we have reduced the minimum self-isolation period to five full days in England.
"This is a balanced and proportionate approach to restore extra freedoms and reduce the pressure on essential public services over the winter.
"It is crucial people only stop self-isolating after two negative tests to ensure you are not infectious."
Good Morning Britain airs weekdays from 6am on ITV and the ITV Hub.
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