Sir Gus O'Donnell was grilled by the House of Lords' comms committee on Wednesday, alongside the PM's director of comms Mike Ellam.
O'Donnell told the committee: 'There's a special advisers' code that would be very clear that it is not what they should be doing.' He said he had not dealt with any complaints against special advisers for attempting to influence civil service press officers.
Meanwhile, Ellam justified the Government's trailing of stories, saying: 'We live in an age where the media want to look forward. They want to report what's about to be said.'