Neighbours are being told to self-isolate because the NHS Test and Trace app is "pinging" people through walls, it has emerged.
Figures show more than half a million alerts were sent through the app last week – the most since records began – raising fears of a "pingdemic", with businesses, transport and schools brought to a standstill.
But The Telegraph has learned that some people are being forced to self-isolate for 10 days despite never having come into face-to-face contact with a positive Covid case.
According to sources close to the Test and Trace app team, the Bluetooth signal used is known to be strong enough to penetrate walls.
That is understood to have been raised as a concern in the initial stages of building the technology – now being reviewed amid concerns it is too sensitive – although Whitehall sources said on Thursday that any changes would not be made for some weeks.
It means people are being forced into isolation because a neighbour whose home they share a wall with has fallen ill.
"We are hearing of anecdotal cases and we do know that it is possible for the signal to travel through walls, although it is weakened," one source said. "The app has been calibrated to try to avoid that happening, but we are reviewing the issue of notifications carefully."
Watch: COVID-19 - Keir Starmer says retuning NHS COVID app to reduce isolation alerts 'like taking the batteries out of the smoke alarm'
The Telegraph discovered hundreds of people complaining on social media about being "pinged" despite not having left their homes. Cases included a carer who had to cancel her father's cancer appointment after her two neighbours tested positive for Covid.
Dr Fiona Sampson, a senior research fellow in emergency and urgent care at the University of Sheffield, told The Telegraph: "My partner got pinged and rang 111 to find out when the contact was. However, he hadn't left the house on the day of the alleged contact.
"We later realised he had been working with his phone on the table, less than two metres away from our neighbour."
Jason Delaney, 39, who owns a bar in Alton, Hampshire, said he was pinged despite not having come into close contact with anyone on the day in question. He added: "I then found out that my neighbour had tested positive for Covid, and we have a standard connecting wall.
"I stayed home for 10 days, like I was told, and that probably cost me around four grand in takings. To be honest, I have lost a lot of confidence in the app. You try to do your bit to stop the infection spreading, of course – but to be told to self-isolate for no reason makes you think 'is this really worth it?'"
A government spokesman would not say how many people were believed to have been pinged through walls, but insisted the number was not large enough to be considered "an issue". The spokesman added: "But we wouldn't say that this never happens." NHS guidance says the Bluetooth signal is reduced through walls rather than blocked entirely, with people on the other side "less likely" to receive an alert.
It comes as the NHS app wreaks chaos, with the latest figures showing that 530,126 alerts were sent in the most recent week – up 46 per cent on the previous week and the highest seven-day total since data was first published in January.
The number is more than 10 times as many as in the week to June 2. In the final week of April, only 39,875 close contacts were identified.
On Thursday night, the chief executive of Rolls-Royce said the firm was "on the edge of a critical situation" and would not rule out shutting down production after a large proportion of its staff were pinged. Torsten Muller-Otvos told The Telegraph: "Cases have gone through the roof, and it is causing havoc."
Watch: What UK government COVID-19 support is available?
Meanwhile, one in five workers in hospitality and retail are self-isolating, NHS hospitals are reporting staff absences of up to 25 per cent and bus and train services are frequently cancelled or delayed due to driver shortages.
Robert Jenrick, the Communities Secretary, said on Thursday that the Government was increasingly worried about the number of people being forced off work by the app.
"We are concerned about absences as a result of being pinged, for example. That is one of the reasons why we do need to move to a more proportionate approach," he told LBC Radio. "The Government is going to be setting out its plans in the coming weeks, so I'm not going to pre-empt those."
The Covid app is based on an algorithm using a weakened Bluetooth signal to track those who have been within two metres of someone with Covid for 15 minutes or more. NHS guidance says the signal is "reduced" through walls and therefore "less likely to satisfy the necessary criteria" if someone is close to a neighbour.
Ministers are concerned that many users are deleting the app after a recent poll showed that 19 per cent of adults had removed it from their phones, with a further 20 per cent saying they would do so after most Covid restrictions are lifted on July 19.
Officials and scientists have been tasked with tweaking the app to make it less sensitive, while double-jabbed contacts of positive cases will no longer need to self-isolate for 10 days from August 16.
Dr Jenny Harries, the head of the UK Health Security Agency, told MPs last week: "We have a piece of work ongoing at the moment because it is entirely possible to tune the app to ensure that it is appropriate to the risk."
A government spokesman said: “There is no issue with the app tracing close contacts through walls.
“The NHS COVID-19 app has been downloaded more than 26 million times, saved thousands of lives and stopped hundreds of thousands of cases by doing exactly what it is designed to do - informing close contacts of someone who has tested positive for COVID-19 they are at risk and advising them to isolate.
"In the context of rising cases it is vital people are aware of their personal risk so they can make informed decisions on their behaviour to protect those around them.”
Case study: ‘The whole thing appears to be a complete nonsense’
A carer who believes she was "pinged" through her neighbour's wall had to cancel her father's cancer appointment after being told to self-isolate for 10 days.
Sue Kent, 54, who runs a pet business in Manchester, received an alert last week saying she had been in close contact with a positive case on Monday.
"That day I paid the window cleaner on the doorstep but I doubt we were in contact for 15 seconds, let alone 15 minutes," she said. "Then I was pinged by the app, and it said I'd been in contact with someone. I thought 'how the hell has that happened?'
"Then my neighbours told me they had tested positive. I live in a semi, and when I'm watching TV they are watching TV right next to me, practically. That's the only way I could have been close to someone.
"I did my duty and stayed at home for 10 days, but I had to cancel my father's appointment because I'm his sole carer. It does make you lose heart in trying to help because the whole thing appears to be a complete nonsense."