The security services may be forced to open classified files to members of the public in a landmark legal ruling next week that will quash government moves to keep them secret.
Norman Baker, the Liberal Democrat spokesman for freedom of information, will be given the right to apply to see an MI5 file believed to be held on him covering his activities during an environmental protest in the 1980s.
The Data Protection Tribunal is expected to rule on Monday that Jack Straw, when he was Home Secretary, was wrong to issue a blanket High Court ban on releasing all MI5 files. The decision may open the door for the release of thousands of top-secret documents.
The ruling will prove a serious blow to the Government, which has argued in secret hearings that public access to files could lead to the release of classified information threatening national security.
But lawyers acting for the MP argued that maintaining secrecy should not be an excuse to hold back information that would not compromise the task of gathering intelligence.
Overturning the ban will mean MI5 will have to release classified documents for the first time in its 92-year history.
The Government is expected to issue a challenge to limit the documents it must release so that operations against terrorists and money launderers, for example, are not compromised.
Campaigners hope the ruling could set a precedent for security matters to be dealt with case by case.
Mr Baker believes that releasing his file will have no implications for national security. The MP was informed by an MI5 source, codenamed "the mechanic", that there was a file on him covering his involvement in legal protests in East Sussex against the upgrading of the A27 while he was serving on the district council.
The security services never confirm or deny whether information is held on members of the public. But Mr Baker applied to see the file under the Data Protection Act that came into force last year and was blocked by the former home secretary, who said the security services should be exempt from releasing internal documents.
MI5 is said to hold 440,000 files, 290,000 of which are on individuals including MPs and peace protesters.
The ruling on Monday will allow Mr Baker to press ahead with his attempt to see the documents. It will also allow other individuals involved in environmental protests or the Campaign for Nuclear Disarmament to apply to see their files if they believe they have been subject to surveillance.Reuse content