The move is only a partial concession because it does not allow their own activities to be criticised. The access is limited to security service files which shed light on the activities of other departments.
The concession is revealed by the Parliamentary Ombudsman, William Reid, in the latest report on his findings as the final appeal process of the Government's open government code introduced 18 months ago.
Mr Reid says the Permanent Secretary at the Home Office has accepted that "the files of the Security Service were open to me if that seemed necessary as part of an investigation even though the administrative actions of the Security Service are not in themselves subject to investigation by me".
MI5 is at present accountable only to the Home Secretary. There is to be a Bill enabling them to take part in fighting organised crime, but it is not clear how their conduct will be questioned as they move out of areas covered by the catch-all "national security" phrase.
Their higher profile in recent years has been largely cosmetic, confirming matters such as the identity of the director-general, Stella Rimington.
In his report on the latest six cases of the 64 referred to him so far, Mr Reid complains that Whitehall still seems widely unaware of the code of practice which is supposed to mean departments answer reasonable questions from the public freely and openly. It was brought in to head off calls for a Freedom of Information Act. There has been no advertising of the code, which means the public is largely unaware of its new rights.
Mr Reid says in his report: "One request for information was badly mishandled because staff failed to recognise the need to deal with it in the context of the code . . . In other cases, it was only after my intervention that staff approached the request positively."
Of the six latest cases, Mr Reid upheld four complaints.Reuse content