Former MI5 agent David Shayler has asked the House of Lords for the right to plead at his forthcoming trial that he acted in the public interest when disclosing security service details to the press.

He is appealing against a Court of Appeal decision in September that he had no right to mount a public interest defence to charges of revealing state secrets.

Mr Shayler is facing trial at the Old Bailey accused of breaching the Official Secrets Act by leaking documents to a Sunday newspaper three years ago.

In May last year, Mr Justice Moses decided it was not open to him to assert that disclosure of documents or information without lawful authority "was or may have been necessary in the public interest".

Human rights

His lawyers have maintained that he would effectively be denied a fair trial and that his human rights would be infringed.

On Monday, Geoffrey Robertson QC, appearing for Mr Shayler, told Lord Bingham and four other law lords that the case raised issues "of importance for press freedom and a democratic society".

The Official Secrets Act would be incompatible with Article 10 of the European Convention on Human Rights, which protects freedom of expression, unless it allowed Mr Shayler a public interest defence, he argued.

If true, Mr Shayler's disclosures were "plainly matters of general public importance", Mr Robertson said.

Illegal activities

Some of them "touched a deep nerve in the British culture of liberty", he added.

Mr Shayler, 35, who was born in Middlesbrough and now lives in London, has always maintained the disclosures and a series of later revelations were made to expose illegal activities and incompetence in the security services.

He claimed in the Mail on Sunday that agents in the 1970s tapped the telephone of Peter Mandelson, later to serve as Northern Ireland secretary, and kept a file on Jack Straw, who is now foreign secretary.

Mr Shayler was exiled in France for three years following his newspaper allegations, and spent four months in a Paris jail.