Christopher Graham, the Information Commissioner, said an unnamed mobile phone company contacted his office to warn him about the leak.
It is claimed thousands of records were sold so rivals could contact owners before their contracts expired to offer them a new deal.
The Information Commissioner’s Office (ICO) has investigated and found the information was passed to several “brokers” for “substantial amounts of money”. It is now preparing to prosecute those allegedly responsible.
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.”
Mr Graham revealed details of the allegations as he backed Government proposals for people who steal and sell on personal data to be given prison sentences.
At the moment, the maximum punishment is a fine.
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,” Mr Graham said.
“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.”
Both Vodafone and O2 said they were not the subject of the ICO investigation.
A spokesman for Orange said: “We can now confirm that this investigation does not relate to Orange (Via Telegraph)