Concerns about the risks involved in implementing a standalone coronavirus contact tracking app in the UK are causing further delays.
A second version of the smartphone software was expected to begin testing on the Isle of Wight on Tuesday, but the government has decided to postpone the trial.
Ministers are considering moving the app to technology developed by Apple and Google.
But countries that test that model are experiencing problems of their own.
Matt Hancock, health secretary, had initially stated that the NHS Covid-19 app would be launched across England - and perhaps other parts of the UK - by June 1st.
But he later said the government had decided it would be better to establish a network of human contract tracers first.
However, the BBC has found that one of the main reasons why the initiative is running late is that developers are having trouble using Bluetooth as a means of estimating distance.
However, they still believe they are in a better position to face the challenge than their counterparts abroad who work under the constraints imposed by the two US technology companies.
Bluetooth handshakes
Contact tracking apps are designed to prevent a second wave of infections by keeping a record of when two people are close to each other and for how long.
If one of the users subsequently tests positive for the disease, the records are used to determine the probability that they infected the other. If necessary, an alert is triggered to prevent further spread of the virus.
The UK has adopted a so-called "centralized" approach, which means that the contact matching process is performed on a remote server. One advantage is that it offers epidemiologists more data to deal with the pandemic. France and India are other countries that have adopted this model.
On the contrary, the "decentralized" approach of Apple and Google runs the games on the phones themselves, for this reason it better protects user privacy.
Poland changed its app from a centralized approach to a decentralized approach on Tuesday. Switzerland, Ireland, Germany, Italy and Latvia are among others to have adopted the design of the technology giants.
Both systems rely on Bluetooth "handshakes" to work.
Number 10 is concerned that iPhones will not always be detected due to a restriction that Apple has imposed on apps that don't adopt its model.
But the UK team has come up with an alternative solution and is more concerned about other limitations of using Bluetooth.
Train problems
Some of these problems were outlined in a study published by Trinity College Dublin last month.
He highlighted the problems with using the received Bluetooth signal strength as a means of estimating distance.
The researchers warned that the signal strength "can vary substantially" depending on:
- how deeply a laptop is put in a bag
- if the signal has to go through a human body to reach the other phone
- whether the two people walk side by side or one behind the other
- if the devices are inside instead of outside
- if the smartphone is surrounded by metal objects
The report highlighted worrying results when the Singapore TraceTogether app was tested.
An experiment inside a stationary rail car found that when users moved from a distance of 3.5 m (11.5 feet) to 4 m, the signals became stronger rather than weaker due to the way where metal objects reflected radio waves.
A supermarket trial also found that the received signal strength was the same if two people walked close or 2 m apart.
Follow-up tests using Apple-Google technology are currently underway.
"Work is ongoing, but preliminary results are broadly consistent with previous observations," said Dr. Brendan Jennings, who was assigned the task of evaluating the effectiveness of the Covid-19 app in Ireland.
Hidden data
The team behind the SwissCovid app from Switzerland is testing independently.
His chief of Bluetooth measurements believes that the problem can be partially resolved by taking a series of readings for a period of at least five minutes.
But he added that Apple and Google have curbed what could be achieved.
"The Google and Apple APIs [application programming interface] it limits the amount of raw information actually exposed to the app, "Professor Mathias Payer told the BBC.
"For maximum utility, we would get all the different measurements, but this has privacy implications."
Apps that use Google and Apple technology can't see the actual signal strength, but rather one of three values, based on the calculations used to normalize the different ways that Bluetooth behaves on different phones,
In contrast, the British team can currently obtain the measurements directly.
The managers believe that an additional advantage of their centralized approach lies in the fact that the data can be processed on the affected server, since it would be too demanding an activity to be carried out on smartphones.
But part of their challenge is to communicate this to Baroness Dido Harding - who runs the broader Test and Trace program - and 10 Downing Street itself.
A prime minister spokesman declined to comment.