More than a million customers have had their personal details “lost” after a data mix-up at a loan firm’s Birstall headquarters.

The Cattles Group, which owns Welcome Finance loans firm, has written to customers informing them that two back-up storage discs with private information about 1.4 million customers have been misplaced.

Marlene Proctor, 31, from Wibsey, was one of the customers who received a letter saying the firm cannot account for her personal details, including bank details, national insurance number, date of birth and address.

The firm claims there is no evidence to suggest the details have fallen into the wrong hands, but Miss Proctor says the ordeal has left her fearing her details could be used by fraudsters.

She said: “It’s meant to be reassuring, but it’s not. No one knows where my details are and who has them.

“What’s going on? It’s hard enough to get loans or get good ratings without some fraudster type stealing your identity and making it even worse.”

In its letter, the firm said an investigation was now under way at its Kingston House offices, where the details were lost.

As well as customers’ details, the lost IT discs included human resource data about staff who are part of the Cattles Group.

A spokesman for the Cattles Group confirmed two tapes were missing from its Kingston House building in Birstall.

It was discovered at the end of November and an investigation was started immediately, a spokesman said.

The loss has been reported to the relevant authorities, including the Information Commission and its regulators the Financial Services Authority.

The Cattles spokesman said: “The storage tapes contain low-level personal data relating to 1.4 million customers, limited to names and addresses for 800,000, but also including date of birth and payment history for 600,000.

"The tapes also include HR data relating to staff in employment with the Cattles Group up to October 2010. A process to inform affected customers and employees is under way. There is no evidence that the information has fallen into the wrong hands or been used maliciously.

“However, Cattles takes its obligations to protect personal data of its customers and staff extremely seriously and we deeply regret what has happened.”