Police were called in to investigate a series of embarrassing Home Office leaks due to fears that national security could be compromised, the UK's top civil servant said today.
Cabinet Secretary Sir Gus O'Donnell insisted he asked Scotland Yard to launch a probe because there was a danger that "quite serious stuff" could have been made public.
Giving evidence to the Public Administration Committee, Sir Gus said: "When we started the inquiry the reason for it was we were worried certain information was getting out that was potentially very damaging to national security.
"To have access to some other things that had come out in the newspapers, the kind of person (who) would have access to that material might also have access to some quite sensitive stuff."
Sir Gus said one thing that raised alarm was a TV interview by then-shadow home secretary David Davis in which he stated that the Tories decided against releasing "half" the material leaked to them.
"When we started the investigation, the reason why we called in the police was not because we were worried about embarrassment, it was because we were worried about more serious things."
He confirmed he was responsible for asking the Yard to investigate, after discussions with Home Office permanent secretary Sir David Normington.
A special private investigator with high security clearance had already looked into the matter by that time, he added.
But Sir Gus insisted it was for the police to "account" for their operational decisions to arrest Tory frontbencher Damian Green and search his Commons office.
"I do not direct the police, I do not tell them who to interview, who not to interview," he said.
Sir Gus also revealed that the Government was considering calling in police to investigate leaks from the Treasury.
The department's permanent secretary, Nicholas Macpherson, was "very concerned" about the security breaches.
Asked whether police could be called in, Sir Gus said: "That is an option."
But he went on: "For him there is a difference... these are market-sensitive cases.
"There is a difference between national security issues and market security issues."
Sir Gus stressed that there were official routes for civil servants to raise concerns and issues, rather than leaking documents.
He said that, based on a statement issued by his lawyers, Home Office official Christopher Galley, who was arrested on suspicion of misconduct in a public office last month, had committed a "clear breach" of the Civil Service Code.
Sir Gus said he had not been aware when the inquiry was launched who or how many people were carrying out the leaking. Investigations were continuing into whether Mr Galley was the only mole, he added.
When someone had shown a "propensity" to leak, there was a danger they would release other information, according to Sir Gus.
Sir Gus said Mr Galley had clearance to see documents up to "secret".
"Things that are secret are very relevant to national security," he said.
He said a number of civil servants had political activity in their past, but insisted that did not mean they were incapable of being impartial and following the service code.
"We don't regard that as a ban to rising through the ranks," he told the MPs.
"If you're saying to me, should we ban anyone with a political past from working in a political office, I wouldn't do that."Reuse content