The mayor of London has called on the government to publish data on the effectiveness of its new contact tracing app, saying he is "frustrated" by the "lack of information".
Speaking to Sky News in Newham, where the app is currently being trialled, Sadiq Khan said neither he nor officials at the London borough were being told how many people had downloaded the app.
"I asked that question today from the government. They can't tell me," he said, adding that he was "frustrated" by the lack of data.
Mr Khan continued: "There is no reason why the government shouldn't share how many people have downloaded the app, or what difference that it has made."
Without this information, Mr Khan said, there was a risk that the app would not gain public trust and its "central purpose" of containing the pandemic would be lost.
He added: "That's why I think it's so important for the people who are running the app to share that data.
"I'm keen to make sure, during this pilot phase, we learn the lessons. Otherwise what is the point of a pilot?"
Asked why the government was not sharing data, Mr Khan said: "I'm told the figures are there, it's not a question of them not knowing."
He had been informed, he said, that the data was still "raw", to which he responded: "That's not a good enough reason as far as I'm concerned, they can clean up the data pretty quickly.
"What I think we should know is, when the app was launched, on a day by day basis, on a week by week basis, how successful have we been in having local residents download the app.
"It's crucial because, if it's the case that we need to advertise more, market more, we need other ways to get confidence, give us the information and we can then see if the trajectory is as we want it to be. We can learn lessons about campaigns that are successful."
A Department of Health and Social Care spokesperson told Sky News: "The trials of the new Test and Trace app are ongoing and we are incredibly grateful to all residents and businesses in Newham for their continued support.
"We will continue to gather data and feedback which will support the national roll out."
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Mr Khan, who has urged Newham residents to download the app, was speaking at the opening of a new testing facility in the borough, one of the areas of the country which was worst hit by COVID-19 in March and April.
The government began the trial of its second contact tracing, which uses technology supplied by Google and Apple, in the borough last week. The first version, which was built independently in the UK, was scrapped in an embarrassing U-turn after it encountered technical difficulties.
The government reported download numbers for the first app, but it is not clear if it is able to publish accurate data on this app's performance, as its privacy-protecting architecture means it does not gather data on, say, the numbers of people who are sent alerts.
No country in the world with an app built on Google and Apple's framework has published data on its effectiveness, although several have claimed their apps are functioning well.
Apple and Google recently expanded their contact tracing system with an update which allows users to receive notifications of potential coronavirus exposure without needing an app.