Banks and pharmaceutical companies are on a secret list of blue-chip firms that hired private investigators who break the law, The Independent has learned.

The revelation that firms from two of this country’s biggest industries may have commissioned corrupt PIs – without facing prosecution – will fuel concerns that corporations potentially involved in the unlawful trade in private information have so far escaped proper investigation

This newspaper has previously revealed that law firms, insurance companies and financial services organisations have used PIs for years to obtain a range of private data.

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Information on the banks and pharmaceutical companies is contained in an explosive list of corrupt PIs’ clients handed to a parliamentary committee by the Serious Organised Crime Agency (Soca). The list of 101 clients also includes some wealthy individuals.

Following weeks of damaging revelations in The Independent, Soca finally bowed to political pressure earlier this week and privately released to MPs the historical details which its investigators ignored for years.

However, the agency has classified the material as secret to safeguard individuals’ human rights and protect the “financial viability of major organisations by tainting them with public association with criminality”.

The decision comes as the newspaper industry is at the centre of the largest criminal investigation in British history over practices including the hiring of corrupt PIs.

Asked this evening if the classified information contained details of banks and pharmaceutical companies, Keith Vaz, chairman of the Home Affairs Select Committee, said: “This affects all manner of organisations.”

Mark Lewis, the lawyer who represents the Milly Dowler family and a long-time scourge of Fleet Street, said: “Consistency demands that the same rules apply to all, whether you run a newspaper, a pharmaceutical company or a law firm.

“As soon as you depart from the equal applicability of law to all, then the law really does become an ass.”

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Trevor Pearce, the director-general of Soca, decided to classify the details of blue-chip companies, in line with Cabinet Office guidelines about sensitive material.

He demanded the list be “kept in a safe in a locked room, within a secure building and that the document should not be left unattended on a desk at any time”.

However, in what would amount to a remarkable snub, the committee is so angry with Soca that it is considering releasing the information under parliamentary privilege.

Mr Vaz said: “We will come to a view as to whether or not we will publish this list. These events took place up to five years ago. Those companies or individuals who either instructed private investigators to break the law or did nothing to stop them must be held to account.”

It is understood other members of the committee are furious that they are being asked to participate in the cover-up. One source said: “This is bigger than the phone-hacking scandal and the committee does not want to be held accountable when all this comes out in the wash.”

Last month The Independent revealed that Soca compiled a dossier in 2008 that outlined how firms, individuals and organised crime bosses hired criminal PIs.

The investigators broke the law to obtain sensitive information, including mobile phone records, bank statements and details of witnesses under police protection.

Soca was analysing intelligence from mostly Scotland Yard investigations that had also failed to prosecute the offenders for the most serious offences – and completely ignored the blue-chip clients who may have profited from their crimes.

The report – which showed the practices went far wider than the newspaper industry – was dismissed by Lord Justice Leveson, who considered it fell outside the narrow terms of reference for his inquiry into the media.

One of five police investigations reviewed by Soca found private detectives listening in to targets’ phone calls in real time. During another police inquiry, the Soca report said officers found a document entitled “The Blagger’s Manual”, which outlined methods of accessing personal information by calling companies, banks, HM Revenue and Customs, councils, utility providers and the NHS.

Illegal practices identified by Soca investigators went well beyond the relatively simple crime of voicemail hacking and also included police corruption, computer hacking and perverting the course of justice.

Meanwhile, in an extraordinary joint admission on the Soca website, Mr Pearce and Commander Neil Basu of the Metropolitan Police admit the agency sat for years on evidence of criminality, until it was finally forced to act in May 2011 by former British Army intelligence officer Ian Hurst whose computer was allegedly hacked by corrupt private investigators.

Mr Hurst told The Independent: “For reasons that remain unclear, the Leveson Inquiry did not touch the sides with regard to the police. In the final analysis, law enforcement agencies are going to have to justify why they conspired for years to protect the offenders and their clients, which extend way beyond the media.”

The joint statement also failed to address why Soca has still not passed all its historical evidence to Scotland Yard, which is currently investigating the crimes that the agency ignored.

Tom Watson, the campaigning Labour MP, said: “Why is the Met Police not in possession of all the information it would usually require to investigate criminal wrongdoing? Why did Soca not give all the physical evidence in the form of the original hard drives to the Met?

“The Yard and Soca need to provide an urgent explanation as to why the latter is still sitting on a bank of data that any decent police investigator would require to do a proper job.”

Rob Wilson, a senior Conservative MP, has written to Home Secretary Theresa May calling on her to sack Mr Pearce and Soca chairman Sir Ian Andrews over their refusal to publish the list of blue-chip clients.

A Soca spokesman said: “Trevor Pearce provided the chair of the committee with further confidential information on 22 July 2013. Soca is unable to comment further on that detail. However, as stated in the DG’s covering letter – which is published on the Soca website – the information provided does not allege, either expressly or by implication, that the individuals and companies named in it, or any individuals working for those companies, have or even may have committed a criminal offence.”

Media experts: What they think

Neil Wallis, former deputy editor of the News of the World

"When I was arrested and questioned over alleged phone hacking, none of the evidence produced was anything remotely other than circumstantial at the very strongest.

"I spent 19 nightmare months unemployable on bail before being cleared. So why is it that executives on the Soca list are not being treated in the same way?

"Because I can’t see the difference between me and a posh lawyer who worked for companies who allegedly paid private investigators to break the law. Except, of course, I’m a tabloid journalist and apparently not a respectable businessman.”

James Clappison, a senior Conservative MP who sits on the Home Affairs Select Committee

"This vast intrusion into peoples’ lives is unacceptable. It appears to be worse than the phone-hacking scandal, yet nothing has been done about it.

"If the police and Soca aren’t going to pursue the matter, the committee should ensure openness and transparency and make this information available to the public.”

Nick Cohen, campaigning journalist

"The police can’t pick and choose how they enforce the law. If hacking is illegal for tabloid newspaper journalists, it should be illegal for big business.

"Soca’s defence that publication might damage company balance sheets goes against the rule of law. The commercial interests of Rupert Murdoch have been damaged by the Leveson Inquiry and the prosecution of his journalists.

"But that is not a reason not to hold the Leveson Inquiry and not to prosecute alleged criminals. The law comes first.”

Professor Charlie Beckett, London School of Economics

"I find it difficult to accept that this should be kept secret, and I’m encouraged by Keith Vaz’s remarks.

"I cannot understand why we shouldn’t know which companies hired private investigators that break the law.

"We know which media organisations commissioned them and I can’t quite see what the difference is here.”

Q&A: What Soca does

Q What is the Serious Organised Crime Agency?

A national policing organisation dealing with serious crimes. It also runs live telephone intercepts on dangerous offenders for police forces in England and Wales.

Q What has it done?

Handed MPs secret details of companies and individuals that its investigators knew for years were hiring corrupt private investigators that break the law.

Q Why did it finally hand over the information?

The Independent exposed the cover-up last month, moving MPs to ask Soca for its confidential details.

Q Why has it not prosecuted the clients?

Soca says it cannot prove the firms knew the private detectives were breaking the law on their behalf. It also says releasing their names into the public domain would breach individuals’ human rights and damage their commercial interests.

Q But aren’t police currently prosecuting journalists for exactly the same thing?

Yes.