Prime Minister Boris Johnson is self-isolating at No 10 after NHS Test and Trace told him he had been in close contact with someone who later tested positive for coronavirus.
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.
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.
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.
You can also use the app to check in at venues – for instance, shops, bars, restaurants or places of worship.
Hospitality venues such as pubs and restaurants will be asked to display posters with a QR code, which app users will be able to scan.
The posters will also go up in communal areas of community buildings such as universities, hospitals and libraries.
Used alongside manual contact tracing, the app will help identify close contacts of a user who tests positive, or visitors to a premises that has suffered an outbreak.
Northern Ireland launched an app in July, while Scotland’s app was launched in September.
People who display coronavirus symptoms and test positive have been contacted by text, email or phone. England’s NHS Test and Trace service calls only from 0300 0135 000.
They are asked to log on to the NHS Test and Trace website and give personal information, including:
Close contacts are:
Contact must have taken place within a nine-day period, starting 48 hours before symptoms appeared.
No-one who is then contacted will be told your identity.
If you are approached because one of your contacts has tested positive, you must stay at home for 14 days from your last point of contact with them.
You must self-isolate, even if you don’t have symptoms.
Others in your household won’t have to self-isolate unless they also develop symptoms, but must take extra care around you regarding social distancing and hand washing.
The prime minister claimed the UK’s test and trace system would be “world-beating”. But it has encountered several problems since its launch in May.
Sage, which advises the government, has said that at least 80% of contacts would need to isolate for it to work properly.
The mobile app was meant to launch in mid-May and form a key component of the UK’s tracing strategy. But the original attempt, which used a ”centralised” approach to storing data, had to be abandoned.
An NHS Test and Trace call does not always mean a pub or restaurant must close. It depends on the circumstances and when the infected person visited.