Dr Farrell also warned that the system was susceptible to ‘relay attacks’, in which an unsophisticated hacker could rebroadcast signals, and that use of the app on Android devices could give rise to privacy concerns.

He said that while the app itself was secure, Android users had to turn on the Google Play Services function to use it, which allows the company to obtain information about them such as email and IP addresses and device serial numbers.

“If you don’t care about Google tracking you, it’s fine,” he said. “Most people don’t care, but for the set of people who do not want Google tracking them but do want the app, it’s a quandary.”

While he emphasised that the app developers and governments were acting in “good faith”, he said he had concerns about people being told to self-isolate for a fortnight based on unreliable readings.

He added: “I would have been happier if this had been proposed as ‘let’s all do this giant experiment together to see if it will work,’ because it’s really unclear whether it will be useful, useless or even negative in some cases.

“If you have a good testing system you can handle the false positives quickly and easily. But if a notification means stay at home for two weeks, that’s a high burden. It makes the whole thing seem more risky, in my mind at least.”

The First Minister on Friday repeated her call for “as many people as possible” to download the app. She said: “This app is a really important way in which all of us can support test and protect in the efforts they are making, but also a really important way for all of us to keep our communities safe.”

A spokeswoman for the Scottish Government said: “The Protect Scotland app, like other apps of a similar nature, uses signal strength to approximate distance between two phones.

“The accuracy of this system has been tested and is considered to be sufficiently accurate in identifying the distance at which someone might be more at risk of Covid-19.

“This technology continues to improve, and we will work in conjunction with governments, developers and Apple and Google to refine the system based on technology developments and close monitoring of performance.”

She added: “The app has also undergone a full system security audit, with advice from the National Cyber Security Centre.

“No-one can see an individual’s data. The Scottish Government has chosen to build the Protect Scotland App on the Google Apple Exposure Notification System as it does not collect information on the user’s identity or location data. We have also chosen not to include any additional features in the App that capture user data. This system ensures that no one sees either their own, completely anonymised and non-identifiable data, or anyone else’s data. 

“The App does not track people or know who they are. Only completely anonymous data is stored in the users mobile phone, and it is only stored in a public registry if the user decides to share their anonymous data.”