The contact-tracing software built by Apple and Google to support public health apps with tracking the spread of coronavirus has been released for the first time.

The tech giants have worked together to create an Exposure Notifications System which can be built into health agency apps, using Bluetooth to track contact with others and alert users when they may have been exposed to the virus..

The Apple and Google software follows what is known as a "decentralised" approach - where the contact data collected stays on a user's device and can be analysed there, which supporters argue increases privacy and reduces the risk of identifying users by de-anonymising data.

In a centralised system, when a user develops symptoms and chooses to declare it, they consent to sending their data to a central server which then analyses which contact events had a risk of virus transmission and therefore who to send an alert to.

Apple and Google said currently 22 countries on five continents have requested access to its software.

The technology giants said the new system was meant to be a supplement to, not a substitute for, traditional contact tracing and should be viewed as an additional tool, not a silver bullet.

"Starting today, our Exposure Notifications technology is available to public health agencies on both iOS and Android," Apple and Google said.

"What we've built is not an app - rather public health agencies will incorporate the API into their own apps that people install. Our technology is designed to make these apps work better.

"Each user gets to decide whether or not to opt-in to Exposure Notifications; the system does not collect or use location from the device; and if a person is diagnosed with Covid-19, it is up to them whether or not to report that in the public health app.

"User adoption is key to success and we believe that these strong privacy protections are also the best way to encourage use of these apps."