The clients list includes law firms, financial services and insurance firms, accountants and celebrities. .
The UK Information Commissioner’s Office (ICO) has launched an investigation into 98 private firms and individuals accused of appointing rogue investigators to collect personal information.
The investigation was launched after it was handed by the Serious Organised Crime Agency (Soca) a list of 98 companies and individuals who are connected to private eyes.
Soca’s operation led to conviction of four private detectives for fraud offences in 2012 after they reportedly gathered information illegally.
The watchdog will now determine whether the firms have violated the Data Protection Act. If found guilty, they would face fines of up to £500,000.
Alleged firms would also face criminal prosecution, while enforcement notices and undertakings would be issued to modify policies or procedures.
ICO said in a statement that the team will look to establish whether the clients fall under the ICO’s jurisdiction. The initial phase of the investigation is expected to take several months.
"We will liaise with our international counterparts where an organisation or individual looks to have breached the Data Protection Act, but is based abroad," the watchdog said.
"We envisage the initial phase of this investigation will take several months, after which time we will publish an update.
"As we are yet to assess the material, and as that assessment may prompt criminal investigations, we will not be publishing the list of clients at this stage."
In July 2013, the UK Home Affairs Select Committee published a breakdown of the clients by firm but did not disclose their individual identity, which included 22 law firms, financial services and insurance firms, accountants and two celebrities.
The list also includes 16 private investigation agencies.