The Information Commissioners Office has confirmed there are 125 alleged victims of the illegal snooping.
Information Commissioner Christopher Graham has refused to reveal the names of firms under suspicion while the investigation is ongoing.
But the BBC reported that he confirmed in a letter to MP Keith Vaz that the investigation covered retailers and legal firms, three insurance companies, two financial businesses, a construction firm and a security company.
The allegations cover the period 2001 – 2009 and 12 of the firms involved are believed to be no longer trading.
The companies are accused of using private eyes to ‘blag’ private information on false pretexts.