UK Bank Halifax has started writing to 13,000 mortgage customers to inform them that some of their details were contained in documents stolen from a member of staff late last week.

The bank says a briefcase including customer account details was stolen from a Halifax employee's car late on Wednesday evening last week. The incident was promptly reported to all the relevant authorities, including the Financial Services Authority (FSA).

'We have taken immediate steps to protect our customers,' said Shane O'Riordain, general manager of group communications. at Halifax. 'It is almost impossible that any financial fraud could be committed with the stolen information. No customer will lose out in the very unlikely event of fraudulent activity on their account following this incident.'

Halifax says the personal data in the stolen briefcase contained mortgage account information only. It did not include any bank account details, PINs, passwords or details of financial transactions.

Around 1,800 of the relevant customer records included name, address, mortgage account number and balance. The remainder of the records – the vast majority – listed the customer's name, mortgage account number and approval status.