When the BBC obtained exclusive pictures of Special Air Service soldiers fighting Islamic State militants in Syria, the Ministry of Defence refused to comment.

British Special Forces are now on the ground in Libya as well - although nobody ever voted in favour sending them, and there has never been a debate in Parliament about it.

In fact, it's a long-standing cross-party tradition that the government doesn't comment on the operations of the Special Air Service, the Special Boat Service, and other Special Forces units.

In this edition of The Briefing Room, David Aaronovitch calls on expert researchers and former military offers to trace the development and current activities of Britain's Special Forces.

Who are they and how many of them are there? Why, when we talk about military operations, do Special Forces not count as "boots on the ground"? And why are their activities so secret?

CONTRIBUTORS:

Mark Urban, Newsnight's defence and diplomatic correspondent

Robin Horsfall, former SAS member

Anthony King, Professor of War Studies, University of Warwick

Emily Knowles, Project Manager of the Remote Control Project, Oxford Research Group

Dr Jon Moran, Reader in Security, University of Leicester

Lt Col Stephen Grenier, author and former US Special Forces officer

PRODUCER: Mike Wendling.