[bbc] Staff at mobile phone company T-Mobile passed on millions of records from thousands of customers to third party brokers, the firm has confirmed.
Details emerged after the firm alerted the information commissioner, who said his office was preparing a prosecution.
Christopher Graham said brokers had sold the data to other phone firms, who then cold-called the customers as their contracts were due to expire.
A T-Mobile spokesman said the data had been sold "without our knowledge".
T-Mobile staff sold personal data