TSB customers complain about fraud alert failures
TSB customers who have had money taken from their accounts say they have not received alerts warning them of fraudulent activity.
Victims have told BBC Radio 4's You and Yours that their accounts have been emptied without them getting a single call or text from the bank.
It is the latest issue to have emerged following TSB's problematic switchover to a new computer platform.
TSB say their fraud detection systems are monitoring people's accounts.
Annette, from East Sussex, had more than £3,500 stolen from her internet bank account. TSB wrote to her apologising for not contacting her about changes that had been made to her telephone number and password as the fraud was carried out.
The letter also acknowledged that multiple, unusual transfers of money out of her account had not been spotted, saying: "We have in place sophisticated systems which are designed to spot fraudulent activity, however these transactions were not flagged by the system.
"We are unable to explain why, but please be assured that we continually update and improve our systems to improve their detection rates".
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'Let down' again
TSB refunded all her money and credited her bank account with an additional £100. But that money - and everything else in her bank account - also seems to have been stolen.
Annette told You and Yours that even though her log-in details and password had been changed she had been affected by another fraud on her bank account.
She said she was "furious" with TSB. "They've let me down again. I've told them 'you can't keep my money safe and I've no choice but to leave'."
Annette says that she did not receive any fraud alert from TSB when her account was emptied for a second time. Other customers are making similar complaints.
One man from Perth said he did not hear from the bank when someone moved about £13,000 from his three TSB accounts then set up multiple payments to transfer everything out.
The migration of data on TSB's five million customers from former owner Lloyds' IT system to a new one managed by current Spanish owner Sabadell created major difficulties for the bank.
Ahead of that move, the bank's chief executive was quoted as saying the new system would make it "so much easier for us to pick up on fraudulent transactions".
Some customers have reported getting fraud notifications from the bank but have then struggled to get any more information.
Susan, from Edinburgh, went into her local TSB branch for help after failing to get through to the fraud team on the telephone.
She had been contacted by TSB after unusual activity had been spotted on her account. She had been promised a call back but had heard nothing more.
Susan says branch staff tried for three hours, but even they could not get an answer from the fraud department.
"I am panicked and appalled because I am left with zero in my bank account. To all intents and purposes I'm destitute and no one is making an attempt to let me know what is happening," she said.
"It is terribly worrying. I've not slept. I have been tearful. I have spent hours by the telephone. There's fraud - I would have expected if not an immediate response then certainly within maybe an hour or so. It is people's life savings here and I don't even know if I am going to get my money back".
Following the move to their new computer system, TSB warned that criminals were specifically targeting their customers with phishing emails and texts.
TSB said no one would be left out of pocket but refused to say if there had been a surge in fraud on customers' accounts or if their computer problems had affected their ability to alert customers to fraudulent transactions.
"Protecting our customers' money is our number one priority and we have a multi-layered anti-fraud framework in place to make sure they don't become a victim of fraud," a spokesman said.
"Our fraud detection systems are monitoring customers' accounts and we're on the lookout for suspicious activity."
You can hear more on this story on the You and Yours programme website.