Terrorists and criminals could gain access to top-secret surveillance information on them if MI5 is forced to open its files to the public, the counter espionage agency is expected to claim in an attempt to block an unprecedented legal move.
The intelligence service will have to release thousands of top-secret documents held on "subversive" individuals if a landmark legal challenge forcing it to disclose the confidential documents succeeds this month. Norman Baker, the Liberal Democrat Home Affairs spokesman, has asked MI5 to give him access to a file which he believe the organisation holds on him.
An internal MI5 source has claimed anonymously that Mr Baker's file includes details of his lawful environmental campaign activities in the 1980s. The tip-off by a serving intelligence officer code-named "the mechanic" included details about his campaign activities that he says only the intelligence services would know.
Mr Baker was refused access by MI5 to the documents after making a formal request under the Data Protection Act.
But with the support of the civil liberties organisation Liberty, he will challenge the decision at a Data Protection Tribunal this month.
If successful, the Government's other spying services, MI6 and GCHQ, would almost certainly have to open their files to public scrutiny. Whitehall sources predict a flood of applications from individuals involved in environmental and peace campaigns.
The tribunal will be held under conditions of secrecy and government lawyers have banned the disclosure of any evidence presented by the Government's intelligence service at a tribunal, or a series of preliminary hearings that have already taken place.
Mr Baker's lawyer, John Wadham, the director of Liberty, is expected to reject MI5's arguments that allowing individuals to see their files thousands of which are no longer active will compromise national security and mean that terrorists will be able to see the results of surveillance activity.
Mr Wadham is expected to say that the refusal to show people what is written about them breaches data protection and human rights law,
MI5 would not have to release information that could compromise ongoing investigations, such as those into terrorists. But if the law is changed, the security services would have to assess whether to release files on a case-by-case basis and lift the blanket ban. Hundreds of files on so-called "subversive individuals" including MPs relate to their membership of lawful campaigning groups in the 1970s and 1980s and have since been closed but are still held in MI5 archives.
The Data Protection Act, which came into force last year, requires greater openness by organisations that hold files on people.
The security services were warned by the Data Protection Registrar that they could face court action if they failed to comply.
But Jack Straw, who was Home Secretary until last week, ruled on national security grounds that MI5 was immune from disclosing the 440,000 files it held.
Mr Baker, the MP for Lewes, said that the three-day hearing, scheduled for 27 to 29 June, was designed to help to break down secrecy barriers.
"This is about establishing the principle that the security services are subject to data protection legislation. They themselves recognise there are occasions when material can be released and indeed have recently made material available to the Public Record Office," Mr Baker said. "How then can they claim that a blanket exemption is appropriate?"Reuse content