An inquest into the July 7 bombings should not investigate the role of MI5 because it would be impossible without handing over documents that would endanger national security, it has been claimed.

The Chamber of Secrets - 'Thames House' MI5's HQ
Lawyers for MI5 told a hearing: "Taken on face value it is difficult to see how that could be done without handing over the keys to Thames House" referring to MI5's central London headquarters.

Neil Garnham QC said that disclosing the material would "impose a cumbersome and costly distraction from the work of protecting national security."

He said the inquest should be limited to how the victims died and not "in what broad circumstances" so that the role of the security services in investigating the bombers before the attacks would not be included.

Lawyers for some of the families have argued that MI5 should have put two of the bombers, Mohammed Sidique Khan and Shehzad Tanweer, under surveillance after they were seen meeting another terrorist planning a fertiliser bomb attack on a shopping centre and nightclub in 2004.

But Mr Garnham warned about hindsight, adding: "When you know what happens, you know Mohammed Sidique Khan becomes a suicidal jihadist, it is easy to draw conclusions."

But, he added, that "entirely disregards the fact that there was intelligence relating to hundreds or thousands of others all of which deserved some or greater attention."

Mr Garnham said the coroner and jury would have to put themselves into the "shoes of the security service" who were considering material on "hundreds if not thousands of suspects."

In order to do that they would need access to the material on such investigations, requiring them to undergo a procedure called "developed vetting" which would be "intrusive and time consuming."

Even then they would not be constrained by the Official Secrets Act not to disclose the material, he added.

Failure to protect sensitive material would "reduce the ability to counter future threats from terrorism," Mr Garnham told a preliminary hearing at the High Court.

The barrister said that the obligation to investigate MI5 had been fulfilled by two reports from the parliamentary Intelligence and Security Committee which had access to "highly sensitive material."

Mr Garnham said the committee, which concluded that MI5 could not have prevented the attacks, was "uniquely well placed to conduct the inquiry" and added: "There has never been so detailed and comprehensive a report on the work of the security service ever made public."

He said allowing the inquest to go beyond the report would mean they were "bound to go deeper and deeper into confidential, sensitive and national security concerns."

The hearing continues.