Researchers were quick to point out that it is too early to draw any concrete conclusions about what the initial figures mean.

Prof Spector usually works with a cohort of 15,000 twins to tease out the genetic factors behind disease. But when coronavirus put a halt to the work, he realised that the group could help scientists understand whether DNA was playing a role in who was getting coronavirus.

He and his team developed an app so that they could track any symptoms of the virus while the twins were in lockdown, and realised it could also be rolled out to the whole of Britain.

"At the moment, there is no alternative system," he said. "I would have expected an NHS or Government version, but there isn't one. This could really help NHS planning, so you could see spots where there are lots of infections rather than just waiting for bombs to fall.

"We will also be able to work out if some of the symptoms are real or not."

Prof Spector said he was amazed at the response and expected more than one million people to have registered by Thursday morning. People are asked to check in daily and report symptoms so the progression of the virus can be monitored.

"Our first analysis showed we're picking up roughly that one in 10 have the classical symptoms," he said. "So of the 650,000, we would expect to see 65,000 cases.

"Although you can have problems of self-selection and bias, when you’ve got big data like this you tend to trust it more. What we're seeing is a lot of mild symptoms, so I think having this data should help people relax a bit more and stop seeing it as an all or nothing Black Death situation.

"Other symptoms are cropping up. Thousands of people are coming forward to say they have loss of taste, and we may start to see clusters of symptoms."