A ruling is imminent on a test case involving a Sussex MP which would allow thousands of people the right to see secret files held on them.

An independent national security appeals panel was yesterday reported to have agreed that a blanket ban on MI5 releasing information was unlawful under the Data Protection Act.

Such a decision would allow people to apply to see files held on them by the security service, although more sensitive information will be withheld.

The move follows a request by Lewes Liberal Democrat MP Norman Baker for MI5 to release a file believed to relate to his activities as an environmental campaigner during the Eighties.

He was tipped off about its existence by an anonymous letter signed "The Mechanic", which was said to contain information only the security service could have known.

Mr Baker was refused access after making a formal request under the Data Protection Act.

He called in lawyers who, during a two-day hearing earlier this year, argued a blanket ban on information violated his human rights.

Mr Baker said a formal announcement was expected this week.

Some files are expected to relate to Foreign Secretary Jack Straw, Deputy Prime Minister John Prescott and Labour MP Peter Mandelson, whose fall from Cabinet office was sparked by Mr Baker's questioning over the Hinduja passport affair.