UK health chiefs are being urged to safeguard people's privacy as they develop an app to help tackle the coronavirus pandemic.
An open letter published by a group of "responsible technologists" warns that if corners are cut, the public's trust in the NHS will be undermined.
And it urges those in charge to be more open about their data-collection plans.
The BBC asked both NHSX - the health service's tech leadership unit - and the Department of Health to respond.
South Korea, Singapore and Israel are among countries that have already deployed apps that can help the authorities track who users have come into contact with, to help model the spread of the virus.
Taiwan has also introduced what it calls an "electronic fence" system that alerts the local police if a quarantined user leaves their home or switches off their handset for too long.
And in Europe, a number of mobile network operators have offered to provide anonymised data about users' movements to help identify potential "hot zones" where the virus might be at most risk of spreading.
The Prime Minister's advisor Dominic Cummings hosted a meeting at Downing Street on 11 March at which dozens of tech industry leaders were asked how they could help develop an app to tackle Covid-19 in the UK. But there has been no formal announcement about what it will do or when it will launch.
"It is not yet clear how data will be collected or used... nor what technical safeguards will be used," says the open letter.
"We are also concerned that data collected to fight coronavirus could be stored indefinitely or for a disproportionate amount of time, or will be used for unrelated purposes.
"These are testing times, but they do not call for untested new technologies."
More to follow