Prof Tim Spector, who runs the Zoe Covid Symptom study, says catching the Delta variant can feel "more like a bad cold" for younger people.

But although they may not feel very ill, they could be contagious and put others at risk.

Anyone who thinks they may have Covid should self-isolate along with members of their household and book a Covid PCR test at www.gov.uk/get-coronavirus-test or by calling 119.

The classic Covid symptoms people should look out for are:

But Prof Spector says these are now less common, based on the data the Zoe team has been receiving from thousands of people who have logged their symptoms on an app.

"Since the start of May, we have been looking at the top symptoms in the app users - and they are not the same as they were.  Fever remains quite common but loss of smell no longer appears in the top 10 symptoms."

The change appears linked to the rise in the Delta variant, first identified in India and now accounting for 90% of Covid cases in the UK.

"This variant seems to be working slightly differently.  People might think they've just got some sort of seasonal cold and they still go out to parties and they might spread around to six other people.

"We think this is fuelling a lot of the problem. The message is that if you are young, you are going to get milder symptoms anyway.

"It might just feel like a bad cold or some funny 'off' feeling - but do stay at home and do get a test."

Similarly, the Imperal College London React study of more than a million people in England - when the Alpha or UK variant was dominant - found a wide range of additional symptoms linked to Covid-19.

Chills, loss of appetite, headache and muscle aches were together most strongly linked with being infected, alongside classic symptoms.

Prof Spector added:

"These other symptoms may have another cause and are not on their own a reason to have a Covid-19 test.

"However, if you are concerned about your symptoms, seek medical advice."