Information Commissioner: ''The penalties... aren't strong enough to stop it''
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".
Mr Graham, who was appointed earlier this year as the watchdog responsible for safeguarding personal information, said the data breach was the biggest of its kind.
He added that the case illustrated why there needed to be a prison sentence to prevent people from selling private data to third parties.
Mr Graham confirmed his office was preparing a prosecution against those responsible for selling on T-Mobile data.
Justice Minister Michael Wills told the BBC that there was a "strong case" for introducing custodial sentences to prevent the trade in illegal data.
Initially Mr Graham had said he would not name the operator involved as it could prejudice a prosecution.
But after phone firms 02, Vodafone, Orange, 3 and Virgin said they were not the subject of his investigation, T-Mobile confirmed it had been.
Danny Shaw, BBC home affairs correspondent
The increasing use of computers and memory sticks to store and transfer data has fuelled what the information commissioner described in 2006 as a pernicious and widespread trade.
Addresses, phone bills, bank statements and health records - they can all be obtained for a price.
The Information Commissioner's report estimated that you could trace the name and address of a telephone caller from their phone number for £75 and check someone's criminal record for £500.
Suppliers obtain information through two routes - by paying insiders to pass it on or by pretending to be someone who has a lawful need for it - known as blagging.
In August a civilian worker at Essex Police was fined for accessing police intelligence databases 800 times and passing on mobile phone records.
Mr Graham said 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 bans the selling on of data without prior permission from the customer and a fine of £5,000 can be imposed following a successful prosecution.
The UK's fourth largest mobile phone company
Has an estimated 16.6 million UK customers - a 15% share of the market
UK workforce of 6,500
Subsidiary of German firm Deutsche Telekom
Plans to merge its UK business with that of Orange
This would create a mobile phone giant with 28.4 million customers
But Mr Graham said that the mobile phone case suggested that people were "driving a coach and horses" through the legislation.
He added: "This is not just about mobile phone companies. It's about private investigators, it's about blagging information from databases for use to put the frighteners on witnesses, attempt to knobble juries, pursue 'nasty neighbour' disputes, interfere in the family courts, difficult divorce settlements.
"Personal data has value and there are people out there exploiting it."
Justice Minister Michael Wills said the government was looking at bringing in tougher penalties to deter the illegal trade in personal information.
He added: "Given the scale of public concern about privacy of their data, I think we have to look at going further and custodial sentences clearly have to be a part of that."
But Conservative justice spokeswoman Eleanor Laing said: "The government's refusal to establish a strong privacy watchdog is nothing short of scandalous.
"We need a beefed-up information commissioner with a full set of punitive strings to his bow, including the power to fine organisations."
Lib Dem Home Affairs spokesman Chris Huhne said the "shameful incident" proved that stiffer penalties "cannot be introduced soon enough".
He added: "This sorry episode questions the government's wisdom in getting communications providers to hoard increasing amounts of information about us."
A spokesman for T-Mobile said the sale of the data had been "deeply regrettable" and that it had been asked to keep it secret to avoid any criminal prosecutions being prejudiced.
He said: "T-Mobile takes the protection of customer information seriously.
"When it became apparent that contract renewal information was being passed on to third parties without our knowledge, we alerted the Information Commissioner's Office.
The spokesman added that the company and the ICO "working together" had identified the source of the breach and that T-Mobile had "proactively supported the ICO to help stamp out what is a problem for the whole industry".
He added: "We were therefore surprised at the way in which these statements were made to the BBC today."
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