Google has fixed an issue that meant the NHS Covid-19 app 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.
Google reassured users no data had been lost.
The issue affected other countries' track and trace apps too.
Users were told not to clear storage in their app, despite some people suggesting that might be a fix.
Rather than solve the problem, that may cause a loss of data and matches from the period the app was not working, according to Google.
In some cases, apps stuck on the loading page could be kick-started by rebooting the phone.
In a statement, Google said: "Late in the evening on 12 January, an issue with the exposure notifications system on Android began causing delays in the checking of potential exposures for those with apps installed.
"We have issued a fix. It may take a few hours for devices to catch up, and in some cases we will work directly with developers to help with recovery. The issue did not cause the loss of any data or potential exposures."
In November, a bug 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.