The NHS Test and Trace smartphone app is being re-launched today with a second round of trials on the Isle of Wight and in the London borough of Newham.
England's beleaguered app, of which the first version had to be scrapped in June, has now been recreated using technology made by Google and Apple.
Officials are rolling out tests to some staff in the NHS and residents of the two areas to test whether it is good enough to use nationwide.
If the app is found to work it will be used alongside the human contact-tracing system which is based on call centres and local councils visiting people's homes.
Bluetooth technology will keep a record of which phones spend 15 minutes within 2metres (6'7") of one another and then alert people if they have been near someone who later tests positive for Covid-19.
Users will also have an 'isolation companion' which has countdown timer if someone has to self-isolate, and will be able to 'check in' to places they visit using QR codes.
They will also be shown what the risk level is in their local area based on the first half of their postcode, with places being categorised as low, medium or high risk.
The app will rely totally on members of the public co-operating, volunteering to let it track their connections and following the instructions it gives them on getting tested and self-isolating.
Despite efforts to iron out flaws in the technology, the Department of Health has admitted that around half of people who are warned they have been near an infected person will actually not have been within the 2m for 15 minutes danger window.
And three out of 10 people who were put at risk - 31 per cent - won't receive a notification at all. In trials it had a 69 per cent accuracy rate at detecting people who had been at risk, and it was 55 per cent accurate at detecting people who had not.
The announcement of developments in the app comes as data on the manual contact tracing today revealed that test results are getting slower again.
Statistics showed this morning that around two-thirds of people who got swab tests (67.4 per cent) received their result within 24 hours, down from 76.9 per cent last week. Prime Minister Boris Johnson had promised to have this figure at 100 per cent last month.
The newest version of the app is being launched after the first attempt was abandoned in June because it did not work on Android smartphones.
The NHS's app — which was originally promised for mid-May and the NHS spent months developing — was unable to spot 25 per cent of nearby Android users and a staggering 96 per cent of iPhones in the Isle of Wight trial.
This was because the Bluetooth system developed by the NHS effectively went into 'sleep mode' when the phone screens were locked and developers couldn't fix the glitch.
It is not clear why the NHS app was so much worse at using Bluetooth to detect other phones than the Apple/Google technology is.
Officials have not explained exactly why or how the new system is better at measuring the distance between two phones, but Apple and Google's own software appears to work significantly better when the phone's screen is locked.
The companies make the phone operating systems themselves so are better able to fit the Bluetooth software around that, whereas the NHS was unable to make a program that could prevent the app going into sleep mode.
The main difference between the two apps is the way they store data.
Both keep a log of who someone has come into close contact with - but the NHS's app would have kept information in a centralised database, while the Google/Apple app is de-centralised.
NHS app: Lists on NHS servers
The NHSX app would create an alert every time two app users came within Bluetooth range of one another and log this in the user's phone.
Each person would essentially build up a list of everyone they have been in 'contact' with. This would be anonymised so the lists were actually just be numbers or codes, not lists of names or addresses.
If someone was diagnosed with the coronavirus all the app users they got close to during the time that they were considered infectious would receive an alert telling them they have been put at risk of COVID-19 - but it wouldn't name the person who was diagnosed.
NHSX insisted it would have deleted people's data when they get rid of the app, but not data uploaded to the NHS server if they or a contact tested positive.
Apple/Google: Contained on phones
In Apple and Google's de-centralised approach, meanwhile, the server and list element of this process is removed and the entire log is contained in someone's phone.
That app works by exchanging a digital 'token' with every phone someone comes within Bluetooth range of over a fixed period.
If one person develops symptoms of the coronavirus or tests positive, they will be able to enter this information into the app.
The phone will then send out a notification to all the devices they have exchanged tokens with during the infection window, to make people aware they may have been exposed to COVID-19.
The server database will not be necessary because each phone will keep an individual log of the bluetooth profiles someone has come close to. These will then be linked anonymously to people's NHS apps and alerts can be pushed through that even after the person is out of bluetooth range.
People can delete their data from this app at any time.
Different Bluetooth technology made by the phone manufacturers Apple and Google themselves has turned out to be significantly better at detecting other phones.
Officials said the app software now reliably detects 99.3 per cent of nearby app users, regardless of what type of phone they have.
Another major difference between the two is that Apple and Google's technology stores the anonymously log of someone's contacts entirely in the phone - it is never shared with anyone else and can be deleted at any time - whereas the NHS's worked on a system which meant it had to be sent to a centralised database.
Officials have changed this to squash concerns about privacy, now insisting the app 'tracks the virus, not people'.
In another improvement to the privacy afforded by the app, it will have a toggle switch for people to turn the contact tracing on or