The form contained sort codes and previous account details, as well as the salary, address and mobile phone number of someone else who had walked into the bank earlier that day to set up a new current account.

Abigail Barnett said she was shocked to discover the form she had been given contained such sensitive data.

She said: "Every detail you could think of was on it, and it's so dangerous because it could easily have got into the hands of someone who wanted to go out and use those details.

"I would have thought the bank had some sort of system where it would be impossible for this sort of thing to happen. The system has obviously got a flaw somewhere if I managed to get hold of all these details."

When 30-year-old Miss Barnett called the manager of the Kingsley Park Terrace bank, she was told to return the form in an envelope marked "private and confidential".

Conservative MP for Northampton South Brian Binley, who set up the All Parliamentary Group on identity fraud after becoming a victim, described the blunder as "an identity thief's dream".

He said: "This is an example of terribly lax security at the bank, and it horrifies me that staff can hand out personal details in such a casual manner. I have always said that banks need to do much more to protect our money and our details.

"Nationwide should, at the very least, discipline members of staff who breach security in this way. They should also apologise not only to the customer whose details have been passed on, and not only to the customer who received them, but to every single one of their customers."

Nationwide Building Society spokeswoman Charlotte Sjoberg said: "Nationwide is extremely sorry that this situation has occurred and we are investigating how the completed application form was available for collection by Ms Barnett.

"We will be contacting both customers to apologise for this error and offering them reassurance that their accounts remain secure."