The NHS Covid-19 app has stopped working on some Android phones, with users reporting being stuck on the loading page.
People expressed concern on social media that it would mean contact data was not being uploaded and users would not receive exposure notifications.
Some experts speculated it was a wider issue affecting other Android apps.
Covid-19: NHS app not working on some Android phones https://t.co/FF7YjDHOuK— BBC News Technology (@BBCTech) January 13, 2021
The Department of Health is looking into the matter with Google, which has not responded to a request to comment.
A Department of Health spokesperson told the BBC: “We are aware of an issue affecting Android users of the NHS Covid-19 app who are receiving a loading screen notification.
“We are working with Google to urgently investigate and resolve the issue as soon as possible.”
In November, a similar problem meant the the app stopped working for many iPhone users, although Apple said it did not think the problem was its fault, because apps in other countries were not affected.
At the time the NHS told users to reset their iPhone location and privacy settings, and to make sure they had the most up-to-date version of its operating system.
The England and Wales NHS Covid-19 contact tracing app had been downloaded more than 20 million times as of 2 December.
Scotland and Northern Ireland run separate apps.