The contact tracing trial on the Isle of Wight is being undermined by people on UK mainland also downloading the technology, members of an ethics board overseeing its development have acknowledged.
Sources close to the development of the Covid-19 tracking technology say the Government may now be forced to rush through a second trial before the contact tracing app is introduced in June.
Two figures close to the ethics advisory board appointed by NHSX have told the Telegraph of scenes of "chaos" prior to the technology's development.
Of most concern to the board is repeated difficulties in obtaining information from Government - including, initially, the questions that would be put to the public to identify symptoms on the app.
Since a smartphone app trial was launched on the Isle of Wight, the Government has said more than half of citizens have downloaded and used the technology.
However, sources close to the technology have cast doubt on those figures. "It's clear that some people who downloaded it were not on the Isle of Wight," one member of the ethics committee said. Doubts have been raised repeatedly in recent weeks over the technology's likelihood of success.
Prof Ross Anderson, a professor of security engineering at Cambridge's Department of Computer Science and Technology, believes the ethics board are being denied critical information which could compromise the safety of the app. "I spoke with the ethics committee and they admitted it," he told the Telegraph.
"The problem with it is that it was perfectly reasonable to say at the start of March let's develop an app because at a time like this you just do everything. But what you have to do is also have a mechanism where people can fail fast and where people can abandon things quickly if they are not working and then try other things."
Ministers acknowledged this week that the tracing app would not be ready by June 1, when lockdown will ease again with the reopening of primary schools and more businesses.
James Brokenshire, the security minister, argues the app was “only one part of the system” and, on Friday, local authorities were handed a new funding package of £300 million from Government to support track and tracing on the ground.
Prof Maggie Rae, the President Faculty of Public Health, and David McCoy, a public health professor at Queen Mary University, London, said in a joint statement that tracing on the ground will be key to success - and the app should just be regarded as an "add-on".
"The use of an App was not a major factor in South Korea’s success," they said in a joint statement to the Telegraph.
"In contact tracing, the human element is all important and the follow-up actions to ensure that contacts of positive cases are alerted and advised on actions, including isolation, to prevent further spread.
"This is the process that will suppress the virus and manage possible outbreaks or ‘hot spots’ of infections. The £300 million investment in strengthening further the new track and trace programme is most welcome as is the recognition of the pivotal role and contribution of local government, and their public health teams, to the next phase of managing the pandemic."
In a statement issued by NHSx, Sir Jonathan Montgomery, chair of the Ethics Advisory Board, suggested he does not share concerns over an alleged lack of information from the Government.
The University College London professor, who previously headed the Nuffield Council on Bioethics, said: "Given the scale of the threat from the pandemic, the NHS needs to act quickly to identify how technology can best be used to protect people. The board has received early sight of papers and had regular briefings from those involved in building the app. This has enabled us to provide advice which has been positively received."
The ethics board was established to "provide independent constructive challenge to the team developing and deploying the NHS contact tracing app", he added.
"Information is shared with the EAB, in good faith, enabling it to provide informed and constructive advice," a Government source said.
"We have worked quickly to build the NHS Covid-19 App because that is what the situation demands, but we would never let that urgency compromise our commitment to clinical safety, transparency, ethics and the law. To claim otherwise is inaccurate and uncorroborated."