NatWest has confirmed that fraud concerns have led it to suspend a feature on a mobile phone app which allows customers to withdraw cash without using a debit card.
The GetCash service was launched in June, but the bank took the step of disabling the feature last Wednesday while it beefs up security following a spate of phishing attacks by fraudsters.
NatWest said that customers who had fallen victim to the frauds would be refunded and anyone who received a phishing email should not reply to it and should contact the bank.
The Financial Services Authority (FSA) said it was aware of the problem and was working with NatWest.
A NatWest spokesman could not give a figure for how many people use GetCash, but said it was a "relatively small number" due to the feature only being launched a few months ago.
He said GetCash would be up and running again "as soon as possible", but NatWest did not expect the service would be ready to return by the end of the week.
GetCash allows users to withdraw up to £100 cash at a time using the app.
Customers are given a code which is valid for three hours and can be used at an ATM to take out cash.
But some NatWest customers have been tricked into filling out their personal details in response to phishing emails, giving conmen enough information to use the service fraudulently.
NatWest said that people generally should be suspicious of emails purporting to be from their bank which asked them to fill out personal financial details which should only ever be given through a secure internet site.
A statement from NatWest said: "We are aware of an increase in fraudsters targeting customers with phishing attacks to gain access to the GetCash service.
"We have currently disabled the GetCash 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 GetCash service."
The BBC said that dozens of NatWest customers had told Radio 4's Money Box programme that they had been defrauded of thousands of pounds collectively by criminals using GetCash.
One listener from London told the programme he had lost more than £950 in total from 11 cash machine withdrawals over three days.
He said that although he had received phishing emails, he had not sent his banking details to anyone.
Another listener from Mansfield told the BBC her husband had lost £2,000.
NatWest has said it already had strong security measures in place and applicants for mobile banking needed to supply a combination of card and personal details.