The Serious Organised Crime Agency has refused to disclose the names of blue-chip companies who commissioned corrupt private investigators who broke the law because revealing them would damage the firms’ commercial interests, The Independent has learnt.

Sir Ian Andrews, the agency’s chairman, told Parliament that publishing the information could “substantially undermine the financial viability of major organisations by tainting them with public association with criminality”.

In an extraordinary letter to MPs, the former senior Ministry of Defence official said the evidence held for years by Soca, which was revealed last month by this newspaper, has now been “formally classified” because the information may breach the human rights of the law firms, insurance companies and wealthy individuals who hired corrupt private investigators.

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The decision to protect the reputation of certain business sectors is in stark contrast to police action against the practice among newspapers.

The MP Keith Vaz, chair of the Home Affairs Select Committee, said he would be writing to every firm in the FTSE 100 and the top 100 legal firms to ask them to declare whether they have commissioned private investigators, and for what purpose.

He added: “Socahas indicated that it is prepared to give the client list to us in confidence. This has still not been received. It is a disappointment that this is yet another document the Committee has had to receive in secret from Soca.

“In view of the public interest, openness and transparency may be the only way that the public can be reassured that no one is above the law and [that] Soca have done all they can to address this issue.”

Sir Ian and Trevor Pearce, the director-general of Soca- dubbed “Britain’s FBI” - were summoned to appear before MPs after The Independent revealed the organisation knew six years ago that law firms, telecoms giants and insurance companies were hiring private investigators hack, blag and steal private information to further their commercial interests.

Much of the intelligence on the blue-chip industries’ employment of criminals - contained in a confidential 2008 Soca report codenamed “Project Riverside” - came from historic Metropolitan Police investigations. Yet almost nothing was done to disrupt the unlawful trade – or target the clients that fuelled the demand.

The Independent has been told the identities of several of the blue-chip clients contained in the material seized by Soca. They include a corporate giant, a celebrity who regularly broadcasts to millions of people, a well-known media personality and a wealthy businessman.

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The Home Affairs Select Committee was angry that Soca withheld the full, unredacted version of Project Riverside during its inquiry into private investigators last year. After its existence was disclosed, Mr Vaz ordered Soca to reveal the list of clients who “hired private investigators to break the law”.

In a series of letters between Sir Ian, Mr Pearce and the Labour MP, published quietly on the parliamentary website, it emerged Mr Vaz made further demands for “all the information Soca holds on private investigators and their links with the police and private sector”.

In his reply, dated 12 July, Sir Ian wrote: “Given the lack of certainty over guilty knowledge on the part of [the] clients, and the impact that any publication might have on those named (recognising the requirement for public authorities to have respect for individuals’ private and family life under the Human Rights Act 1998), together with the possible prejudice which any publication might have on ongoing criminal investigations and future regulatory action, the list of…clients which Soca has created following your request has been formally classified.

He said: “The fact that they have been identified does not mean that they placed their instructions in the knowledge that the private investigators or their agents would act unlawfully.”

Later he added: “This reflects the fact that the information it contains, if published, might prejudice individual security or liberty, impede the investigation (or facilitate the commission) of serious crime or substantially undermine the financial viability of major organisations by tainting them with public association with criminality.”

One of the key hackers mentioned in Project Riverside has admitted that 80 per cent of his client list was taken up by law firms, wealthy individuals and insurance companies. Only 20 per cent was attributed to the media, which was investigated by the Leveson Inquiry after widespread public revulsion following the phone-hacking scandal.

Neil Wallis, the former deputy editor of News of the World, said: “This is completely outrageous. It’s one law for the rich, another for the not-so-rich who also happen to rattle the cages of the powerful on occasion.”

Scotland Yard revealed last week that the cost of the combined inquiries into newspapers - Elveden, Weeting and Tuleta - were expected to cost nearly £40 million up to their expected conclusion in April 2015.

Many at senior level at the official information watchdog are deeply frustrated by the police and Soca’s failure to tackle the blue-chip clients of criminal private investigators such as law firms and banks.

A senior source at the Information Commissioner’s office said: “It is market forces. If you don’t cut out the demand, it won’t stop. The City drives the corporate spooks but the big boys always escape. We should be saying to the clients that if you buy this information, the financial and reputational damage will be so great, it wouldn’t be worth doing it.”

In a sign of the gravity of the situation, it is understood Soca has launched a frantic hunt for leaks inside its organisation since The Independent revealed that it had sat on evidence of widespread blue-chip hacking for years. It is understood that staff have also been reminded of their obligations under the Official Secrets Act after The Independent published the story.

Q&A: Why Project Riverside matters

Q. What is Project Riverside?

A. It is the name of a review by the Serious Organised Crime Agency into investigations by both Scotland Yard and the Information Commissioner into the murky world of criminal private detectives between 2003 and 2007.

Q. Why is it so important?

A. The eight-page report shows detailed police knowledge of criminal PIs working for industries besides newspapers, such as law firms and insurance companies; yet almost all of them were never brought to justice.

Q. Why is it so sensitive?

A. It shows that police knew of widespread criminality among PIs throughout the period when the Met failed to take proper action against the News of the World. Project Riverside reveals some offenders placed eBlaster Trojans on victims’ computers, yet action was not taken against them.

Q. Why is Fleet Street so angry about it?

A. Project Riverside shows that other industries besides newspapers have engaged for years in the unlawful trade in personal information, yet none has been prosecuted for commissioning illegal acts.