A quarter of Britons are being targeted by phone fraudsters more than once a week, an investigation has found.
Half of these have then been contacted by cybercriminals posing as telecoms security engineers from companies like BT and Microsoft.
The crooks try to trick their victims into downloading “remote access” software so they can hack into their computers.
Consumer group Which? says increasing numbers of fraudsters are using the “cheap” technology to exploit people over the phone and plunder their bank accounts.
One victim who lost £85,000 in the “remote access” scam says she “felt sick” after realising she’d been conned.
The woman, called Claire, was contacted by someone claiming to work for BT.
She said: “He said they were concerned about the number of security alerts on my account and talked me through some software.
“This was my big mistake - I didn’t realise it allowed a third party not only to see what was on my screen but to control it.
“They moved money from my savings accounts to current accounts and removed a huge amount of money in regular tranches.”
A poll of Which? members found eight in 10 have received what they believed to be scam calls. But it says there is a lack of public awareness of how the software works - something fraudsters exploit.
A spokesman said: “Once they have you on the phone, scammers are after something - whether it’s personal data to hack your accounts or to convince you to send them money.
“Cheap tech is enabling them to do that, increasingly through the use of remote access programmes. This software allows someone to access your computer over the internet and is used by legitimate businesses.
“But a criminal, perhaps posing as an engineer from BT or a technical adviser from Microsoft, might also request that you download and install these programmes, claiming they will fix a spurious problem or check your system for non existent viruses.”
The spokesman added: “Once they have penetrated your device, the scammers can do as they please - and may reconfigure the software so they can access it whenever they want.
“They may start running programmes that show copious amounts of data while we’re still on the phone which appears to support the false notion that they are carrying out tests on your computer or downloading other software in the background - but what they’re really doing is scraping information from your machine.”
Which? says the technology can be used by legitimate IT companies but they will never call you out of the blue.
The probe follows a warning from industry experts UK Finance last summer about a surge in calls targeting bank customers with a recorded message asking them to validate or block a request to transfer cash before inviting them to speak to a fraud advisor - usually the scammer.
Which? head of money Gareth Shaw said: “Hundreds of millions of pounds are lost to fraudsters every year, and the rise in scams targeting home phones and the sophisticated tactics used to prey on their victims is alarming.
“While the telecoms industry has established initiatives to tackle nuisance calls and phone scams, the Government, the police and sectors such as banking and the online world must also work closely together to deliver a coordinated plan that finally halts worsening fraud rates.”
