Sir Gus is head of the civil service
The UK's top civil servant says he remains concerned that someone prepared to leak data that could damage national security is still at the Home Office.
MPs have been asking Sir Gus O'Donnell why he decided to ask police to investigate Home Office leaks to Tory frontbencher Damian Green.
He said it turned out those leaks had not been a national security matter but up to 40 others in 2005-2007 had been.
The police investigation failed to find the source of those leaks, he said.
A cross-party committee of MPs is investigating how police were allowed to search shadow immigration minister Mr Green's Commons office without a warrant in November 2008.
The Cabinet Office has been criticised for calling in police to investigate leaks - on the grounds that there had already been "considerable damage to national security" as a result.
A later review of events concluded the leaks which former Home Office worker Christopher Galley passed to Mr Green had been embarrassing for the government, but were not matters of national security.
But Cabinet Secretary Sir Gus told MPs investigating the events the Cabinet Office letter had been right - as there had been "up to 40" leaks "potentially from the Home office" between 2005 and 2007.
Some of those "involved some leaks of highly sensitive material of importance to national security" and some "potentially" involved breaches of the Official Secrets Act.
These had followed a "very similar" pattern to the Christopher Galley leaks - and they hoped by asking police to investigate they might lead them to the earlier, more serious leaks.
"The investigation was not successful in discovering the source of the national security leaks, I remain concerned about this," he told MPs.
He quoted an interview by former shadow home secretary David Davis - who told the BBC last year that "in about half the cases" of information passed to the Conservatives, they decided not to publish - sometimes for national security reasons.
Sir Gus said that showed there was much more information being leaked: "That is very worrying and I remain worried."
He added: "At the moment we have no new information to guide us on that."
But the committee questioned his rationale and suggested there may have been a degree of "hyperbole" in Mr Davis's comments.
Sir Gus agreed that the leaks Mr Galley admitted to were not national security matters and should have been dealt with as a "civil service code issue" and internal disciplinary procedures.
He said he might be reluctant to ask the police to investigate in future because of "the extent to which you completely lose control when you hand over to the police".
He said he had not known an MP was about to be arrested, or that it was done on the basis of "things that didn't actually get into the national security area at all".
"If I had been consulted .. I would have said: 'If it was on the basis of that they should not have gone anywhere near an arrest'."
"We did not determine the conduct of the investigation, nor were we consulted by the police on the decisions to make arrests."
Ashford MP Mr Green was never charged but was held for nine hours by counter-terrorism police in November 2008 as part of the leaks investigation.
An official inquiry into events later ruled the police action had not been "proportionate" and the leaks had never threatened national security.
Christopher Galley, the civil servant arrested for leaking documents to Mr Green, was also not charged but was later sacked for gross professional misconduct.