Because of the error, thousands of people were not told to quarantine even when they should have been, according to the Sunday Times.

The UK originally planned to use its own app, sidestepped the Google and Apple system, but reversed course in June. It was built using Google’s Bluetooth low Energy-based system, which monitors nearby phones.

The app was originally built to recommend anyone closer than 2 meters for more than 15 minutes to someone who later tested positive should quarantine. Before the launch, it was adjusted to take into account when the sick person’s symptoms began.

People tend to have high levels of virus in their nose and throat. Levels stay high the first few days of symptoms, and then drop off. If people were in contact with a sick person outside of that window, the app would consider the interaction less risky.

The adjusted app calculates new risk levels. But the thresholds at which a person would get an alert were left unchanged. Without the updated thresholds, a user could have spent up to 15 minutes with a less-contagious person without getting an alert.

The error was discovered when the engineering team added another update, which could more accurately manage exposures at a distance over a meter away.

A UK department for health and social care spokesperson said the app was’the only app in the world using the latest google-apple technology to better gauge distance to identify those most at risk’.