The Health Foundation said failure to reveal how the technology has performed during trial stages, risks denting public confidence as its success depends on uptake.
The coronavirus contact tracing app is set to be released across England and Wales on Thursday to support the NHS Test and Trace effort, following months of delays, technical hitches and privacy issues.
It uses Bluetooth technology to keep an anonymous log of those in close proximity to a user and can notify them if someone who was near them later tested positive for coronavirus.
The latest version of the app has been trialled on the Isle of Wight and in the London borough of Newham since mid-August.
The charity is concerned people have yet to see the results of these pilots and is calling for greater transparency around the development.
It also wants assurances the technology will not exacerbate existing health inequalities, leaving some people at greater risk of coronavirus than others.
“With a virus that is transmitted as quickly as Covid-19, the automated contact tracing that the app promises could prove invaluable in reducing its spread,” said Josh Keith, a senior fellow at the Health Foundation.
“Also, the additional features of the app, such as booking a test, reporting symptoms or checking the risk level in postcode district could provide a helpful single source of Covid-19 related advice and support.
“However, for any major, nationwide public health intervention it is important the Government publishes evidence that it is effective and ready for mass rollout in advance of its launch.
“This is key for building confidence in the app as people will want to know that it will benefit them and their communities.
“But any data on the pilots that took place in August have been notably absent, leaving major questions over the app’s effectiveness unanswered.”
The Department for Health and Social Care (DHSC) responded saying that trials have shown the app works accurately and responsively, with positive feedback from users.
Major mobile phone operators have also committed to “zero-rating” all data charges, meaning customers will not be charged for data when using the app.
“Ensuring a wide range of people download and use the app is essential,” a DHSC spokesperson said.
“The NHS Covid-19 app forms a central part of NHS Test and Trace in England, and works alongside traditional contact tracing services and testing, to help individuals to understand if they are at risk of infection so they can take action to protect themselves and their communities.
“We have spoken with groups with protected characteristics, such as age, ethnicity and disability, those experiencing health inequalities and those groups particularly impacted by coronavirus and the app and supporting material will be available in multiple languages.”