A new app to help trace the spread of coronavirus hit the top of apple and googles app stores on its first day, in a promising start for project that has been marred by delays and confusion.

The free nhs app launched on thursday after a four month delay and a complete overhaul of its design.

The high number of first-day downloads means the app may clear its first main hurdle: persuading enough people to use it so that it makes a difference.

Research by oxford university, held up by the designers of the app, suggests that at least 15 per cent of the population needs to have the app on their smartphone to make the system effective. earlier tests on the isle of wight and in newham, east london, saw download rates of 30 per cent and 10 per cent respectively.

In france, a report this month suggested that the national covid tracking app had been downloaded only 2.4m times and uninstalled 700,000 times as people lost patience with it. germany has seen the highest level of adoption of any country for its app, with 18m downloads.

Matt hancock, the uk health secretary, sought to play down the significance of getting the app to scale. even if only two people download the app and they came into close contact, and one tested positive then it would work for the other, he noted.

Reaching a sufficient level of adoption will be a challenge not only because of the chequered history of the project, but also because it only works on smartphones built in the past four years or so, which are equipped with bluetooth low energy.

Only 79 per cent of the uk population has a smartphone, and only half of these are compatible with the app, according to scott mclachlan, a postdoctoral research assistant in electronic engineering and computer science at queen mary university.

The most vulnerable groups to covid-19, the elderly and those from lower socio-economic backgrounds, are the least likely to have a compatible smartphone, with only 9 to 18 per cent of over-55s having such a phone, he added. there are insufficient numbers to create a core user group even approaching sufficient numbers to make it worthwhile, he said.

Zhlke, the swiss it firm that helped to develop the app, has built it on a system developed by apple and google that protects privacy and uses low-energy bluetooth. as a result, the app uses between 2 to 5 per cent of a full battery charge, depending on factors such as the age of the phone and how many charge cycles it has been through, said wolfgang emmerich, director at zhlke.

A previous version of the app, which has now been abandoned, was not accurate, capturing three-quarters of contacts between people with android phones but only 4 per cent of contacts between iphone users.

The new app is better at registering contacts between phones, but is still prone to false readings because of the nature of the bluetooth system.

Bluetooth can connect through walls and other partitions, said pete fussey, a professor of sociology at the university of essex. this means people in high density accommodation... have increased likelihood of being misidentified.

Its not an exact science, admitted mr emmerich, anybody who claims that they can achieve 100 per cent accuracy in distance measurements is not telling the truth. he said false positive rates would be the same as other apps using the apple-google system.

Senior officials involved in the new app say privacy is paramount. users have only to give the first part of their postcode equivalent to about 8,000 households and all data are held only on the user's phone where it can be deleted at any time.

We have built in and google and apple forced us to build in privacy by design, said mr emmerich.

But brent mittelstadt, a british academy postdoctoral fellow in at the oxford internet institute, noted that the privacy baked into the apple and google system may make it more difficult to analyse data, such as assessing the true number of false positives and negatives.

The app also includes a function to scan qr codes at venues, to help bolster the tracing system. this will leave a data trail at venues that may be vulnerable, said jim killock, executive director of the open rights group. this lingering hole in the system seems to be completely unaddressed.

Mr emmerich said interoperability with other countries systems may become possible in the future. this is a lot easier given were using the same [interface] that people in countries you may be travelling to are, he said.

Whats required is a mechanism to share information reliably as to which [users] have experienced an infection internationally, he said, describing it as one of the next steps.

Last month the european commission announced an interoperability gateway service to test out linking national apps from six eu member states.