HSBC's voice recognition security system has been tricked by an account holder's twin brother mimicking his speech.
The system is used by half a million customers and when it was introduced last year the bank's head of retail banking said that "just like your fingerprint, your voiceprint is unique". HSBC says it analyses 100 behavioural and physical vocal traits.
The bank said that it would look at ways to make the system more secure after Dan Simmons, a BBC reporter, opened an account for the Click programme, which his non-identical twin brother was able to access at the eighth attempt.
The breach in the telephone banking security did not allow the impostor to withdraw money but he had access to balances and recent transactions and could move money between accounts.
Joe Simmons, who fooled the system, said: "What's really alarming is that the bank allowed me seven attempts to mimic my brother's voiceprint and get it wrong, before I got in."
Technology experts said the case highlighted that biometric systems, which are used widely by banks and other organisations, are not infallible, and should only be used alongside other layers of security. Researchers have previously shown how easy it is to trick fingerprint ID systems.
A spokesman for HSBC said: "The security and safety of our customers' accounts is of the utmost importance to us. Voice ID is a very secure method of authenticating customers. Twins do have a similar voiceprint, but the introduction of this technology has seen a significant reduction in fraud."
The bank also suggested that the reporter's brother would have had access to personal information that a stranger would not have.