The rollout of the new contact tracing app in England and Wales has been hit by complaints from some users that they are unable to download it because of the age of their mobile phone.
Users took to social media on Thursday to express their frustration at being unable to use the app because their device was not compatible.
The NHS Covid-19 app requires Apple iPhone users to be running the iOS 13.5 version of the firm's software, released last year but available to devices several years older, while users of Google's Android require version 6.0, which was rolled out in 2015.
Such a cut-off point exists because the technology used to power the app, including how Bluetooth is used to carrying out contact tracing, is only available in the later versions of smartphone software and cannot be supported on older devices.
According to Apple figures, 92% of all iPhone devices it has introduced in the last four years globally are running at least iOS 13 – meaning they would be able to download the app.
An iPhone running iOS 13 is able to upgrade to the 13.5 version required to access the app.
However, iOS 13.5 is not available on the iPhone 6 and 6 Plus – devices which were first released in 2014 – or any earlier iPhone handsets.
A number of recently released Huawei smartphones, including the Mate 30 series and the P40 series, are also not currently able to download the app because those devices do not have access to Google's app store.
The app has been built on a framework developed by technology giants Apple and Google.
Health Secretary Matt Hancock said on Thursday the "vast majority" of people had the right software, adding that some may need to upgrade their phone's operating system in order to access the app.
The Government has also said that anyone unable to use the app should continue to use traditional contact tracing services provided by NHS Test and Trace or NHS Wales Test, Trace, Protect.
Google has not provided figures on the number of users running versions of Android that would support the app.