NatWest bank has admitted that fraud is behind its decision to suspend a mobile phone application which lets customers withdraw cash without a debit card.
Dozens of customers have told Radio 4's Money Box programme they have been defrauded of thousands of pounds by thieves using the Get Cash system.
The Financial Services Authority (FSA) says it is aware of the problem.
NatWest says the fraudsters are using phishing attacks to gain the details needed to access the system.
It will refund customers, but so far the bank has not been able to explain fully how the frauds have been perpetrated.
NatWest only recently launched Get Cash, along with another system called Emergency Cash, which involves calling a phone operator.
Both systems generate a security code allowing customers to withdraw money from cash machines without a card.
On Saturday, NatWest said it had suspended the Get Cash system as part of a planned update.
However, it has now admitted the situation is serious.
"We are aware of an increase in fraudsters targeting customers with phishing attacks to gain access to the Get Cash service," the bank said.
"We have currently disabled the Get Cash service while we increase the level of security required.
"We will enable the service again once this change is complete. We will refund any customers who have suffered loss as a result of fraud via the Get Cash service," the bank added.
'Gesture of goodwill'
Tim, a Money Box listener from London, lost more than £950 after he found that fraudsters had taken money from his account in August using a security code downloaded using a NatWest app.
The money disappeared in 11 cash machine withdrawals in just three days, each one not exceeding the £100 limit NatWest imposes per cardless cash machine withdrawal.
Tim had never heard of the Emergency Cash nor Get Cash systems.
NatWest quickly accepted he had not made the withdrawals and Tim thought a refund would be straightforward.
But the bank then accused him in a letter of giving his personal details to a fraudster via a phishing email.
Although Tim has received such emails, he has never sent his banking details to anyone.
After Money Box contacted NatWest, it said it would refund Tim the £950 as a gesture of goodwill.
Others have lost greater sums. Sophie, another listener from Mansfield, told Money Box her husband had been defrauded of £2,000.
NatWest said it had strong security measures in place for its Emergency Cash and Get Cash withdrawal systems.
"We are fully committed to the prevention of fraud and have stringent security procedures in place in this regard," it said.
"The Get Cash app is only available through the mobile banking app, not as a standalone application.
"Enrolment for the app requires the customer to first register for online banking, then mobile banking and can then proceed to download the app for their mobile," the bank explained.
NatWest said that applicants for mobile banking needed to supply a combination of card and personal details, as well as their online banking number.
A letter is then sent to the customer's home address, alerting them to the fact that mobile banking has been activated.
Tim and the dozens of other customers who have contacted Money Box still do not know exactly how fraudsters were able to obtain these details and NatWest has not told them.