Video report by ITV News Health Editor Emily Morgan
More than one million people have downloaded the government's contact-tracing coronavirus app in England and Wales on the first day of its release.
By using Bluetooth technology, the NHS Covid-19 app tells users to self-isolate for 14 days if they have come in contact with someone who has tested positive for the disease.
The app, which is available on Apple and Android, also has a check-in QR scanner which alerts users if a venue they have visited has had an outbreak of the disease.
The government is urging people over 16 to download the app to help slow the spread of the disease.
Thursday's release comes after months of delay after the Government abanonded plans to set up its own Covid-19 trace app in June to eventually swtich to technology used by Apple and Google, costing months of progress.
Many governments have tried with varying success to make apps that will use Bluetooth to tell users when they've been near someone who has Covid-19.
Scotland launched a similar app on September 10.
Watch how the new app works
Professor Pete Fussey, from the University of Essex and Research Director of the ESRC Human Rights, Big Data and Technology Project, said the app has several hurdles in its way before it is effective.
For it to be effective the app cannot exist in a vacuum, it will only be useful if there is an effective testing regime functioning nationwide.
Professor Fussey also said the key Bluetooth element may be "weak".
He said: "Bluetooth can connect through walls and other partitions.
"This means people in high density accommodation have increased likelihood of being misidentified.
"These instances are also likely to occur in high density urban areas where health services are most likely to be under strain."
The app will also require large numbers of people to download it to work effectively.
David Bansall, scientific advisor to the Test and Trace programme explains the function of the app
Professor Fussey said low levels of adoption had been a problem for apps worldwide.
Addressing why people should follow the instructions of the NHS Test and Trace app launched on Thursday, Boris Johnson said: “The crucial thing is for people to follow the guidance, and when you’re contacted by NHS Test and Trace and told that you’ve been in the presence of somebody with coronavirus – we’re going to give you support to help you through the period of self-isolation.
“You should do that because that’s the best way to stop the spread of the virus and just bear in mind now that there are also fines for people who don’t follow the guidance and that is what I’m trying to say today."
He said: What I’m saying to people is… if you’re told that you’ve come into contact with somebody – an index case of coronavirus – then you should take the action that they tell you to do.
“Nobody wants to put these fines in place, and this is the last thing any of us want to see in this country – a freedom-loving country – but we have got to work together now to suppress the virus, allow education and the economy to continue – that’s the balance in this package.”