Coronavirus: How does Covid-19 test-and-trace work?
Baroness Dido Harding, the head of NHS Test and Trace, is self-isolating after receiving a notification from its app.
Last week her husband, Conservative MP John Penrose, was also told to isolate by the app, while Prime Minister Boris Johnson is self-isolating at No 10 after NHS Test and Trace contacted him.
Mr Johnson spent about 35 minutes with MP Lee Anderson, who lost his sense of taste the next day.
Refusing to self-isolate when told to is now illegal in England, with fines of up to £10,000.
How do I download the app?
You can download the app on a smartphone - but not on tablets, smartwatches or other devices.
To get started, go to Android's Google Play or Apple's App Store and search for "NHS Covid-19".
Your phone must have Android 6.0 (released in 2015) or iOS 13.5 (released in May 2020) and Bluetooth 4.0 or higher. That excludes the iPhone 6 and older versions of Apple's handsets. Some more recent Huawei phones will not load the app either.
What can the app do?
The app can detect when a fellow app user is nearby.
When two phones running the app are near each other, they will make contact through Bluetooth.
If they are close for a long enough time, and one of the two owners later shares a positive coronavirus test via the app, then the other will receive an alert.