A software setting error in the NHS Covid app has meant thousands of people were not told to self-isolate after they came into people infected with the coronavirus.

Software developers behind the app have admitted its algorithm has since been updated – weeks after it was first released on Sep 24. The app has been downloaded by more than 19 million people in England and Wales.

It uses Bluetooth signals to track who an individual comes into close contact with and sends an alert to those people if they become infected with Covid-19.

But on Thursday developers behind the app said an update to the “risk threshold” had been made to take into the account the element of “infectiousness”.

“The ‘risk threshold’ was due to be lowered, but this change did not take place at that time,” wrote Randeep Sidhu and Gaby Appleton in a blog post.  

“The updated version of the app addresses this by lowering the threshold at which users are deemed to be at risk of having caught the virus and alerted to self-isolate. 

“The update to the risk threshold is expected to increase the number of people asked to self-isolate by the app, having been in close contact with someone who has tested positive.”

The result of this software setting means users who should have received an alert telling them to self-isolate were not contacted, according to The Times.

The paper quoted a source saying “shockingly low” numbers of users had been sent warnings, and that users of Android devices were among the worst hit.

The smartphone app has experienced technical issues since its launch after months of setbacks. 

In October the government was forced to upgrade the app after users received alert messages suggesting they had been exposed to the virus.

In many cases, the so-called “ghost notification” would simply disappear once clicked on and was not followed by any official confirmation of contact with others who carry coronavirus. 

“This was confusing for app users who would receive a ‘possible exposure’ message,” Sidhu and Appleton admitted in the same post.

“We recognise this is still an inconvenience and cause for concern for some app users.”

A DHSC spokesperson said: “The NHS Covid-19 app is the only app in the world using the latest Google/Apple technology to better gauge distance to identify those most at risk, and is deemed ‘excellent’ by international standards.”