Staff at one of the UK's major mobile phone companies sold on millions of records from thousands of customers, the information watchdog says. Christopher Graham told the BBC that brokers had bought the data and sold it on to other phone firms, who called the customers as contracts neared expiry. The suspected trade emerged after the firm alerted the watchdog. Mr Graham is planning to prosecute those involved. He said he had not named the company in order not to prejudice a future trial. Mr Graham, the Information Commissioner appointed earlier this year, said the case he was now preparing illustrated why there needed to be a prison sentence to prevent people from selling private data to third parties. Search warrants Mr Graham told the BBC that investigators had been working with the company after it reported suspicions of an unlawful trade in customers' data. The team from the Information Commissioner's Office obtained search warrants to enter premises and have also interviewed employees. Mr Graham said: "Many people will have wondered why and how they are being contacted by someone they do not know just before their existing phone contract is about to expire. "We are considering the evidence with a view to prosecuting those responsible and I am keen to go much further and close down the entire unlawful industry in personal data. "But, we will only be able to do this if blaggers and others who trade in personal data face the threat of a prison sentence. "The existing paltry fines… are simply not enough to deter people from engaging in this lucrative criminal activity. The threat of jail, not fines, will prove a stronger deterrent." The Ministry of Justice has been consulting on tougher penalties for illegal trade in personal information. The Data Protection Act banned the selling on of data without prior permission from the customer - but Mr Graham said that the mobile phone case suggested that people were "driving a coach and horses" through the legislation. "More and more personal information is being collected and held by government, public authorities and businesses," said Mr Graham. "In the future, as new systems are developed and there is more and more interconnection of these systems, the risks of unlawful obtaining and disclosure become even greater. "If public trust and confidence in the proper handling of personal information, whether by government or by others, is to be maintained effective sanctions are essential." Source: BBC