A former head of personnel at MI5 has told the David Shayler trial that all employees owed a life-long duty of confidentiality to the service.

Mr Shayler, who is accused of leaking secret information to the press, denies three charges of breaking the Official Secrets Act.

Brown paper and screens were used to prevent the press and public in the courtroom from seeing the MI5 agent, referred to during the trial as Witness B.

I am trying to find out if I was the subject of an investigation

David Shayler

The judge, Mr Justice Moses, told the jury: "Mr Shayler knows who he is, but he still works for the security and intelligence service and it is right that he should not be named.

"Because he is still working for the service, his future use might be blown."

Witness B was the first witness in the case.

He is one of four MI5 officers who will give evidence anonymously.

The prosecution alleges that Mr Shayler copied 28 files when he left the service in October 1996 and handed them over to the Mail on Sunday newspaper, a year later.

Novel plans

He received total of around �40,000.

Witness B said Mr Shayler had signed the Official Secrets Act three times during his five years service.

The witness said he had rang Mr Shayler to remind him of his duty of confidentiality after hearing he was writing a novel.

He said: "Mr Shayler acknowledged that the book he was writing was remotely connected with his work with the service."

Witness B said Mr Shayler had assured him that he would submit the manuscript for authorisation before it was published.

He had asked if there was interest in the book and Mr Shayler told him he had approached a couple of literary agents "who had not been particularly interested in his ideas".

Cross-examined by Mr Shayler, who is defending himself, Witness B said he did not remember how he found out about the novel.

Secret mark

When the judge asked Mr Shayler why he was asking that, the former agent replied: "I am trying to find out if I was the subject of an investigation."

Witness B denied he had contacted Mr Shayler because he feared he would be the subject of disclosures in the book.

Witness B was followed by Witness A who spoke with an apparent public school accent.

He said he was the former head of the counter-subversives branch where Mr Shayler worked for a while.

Asked by Mr Shayler if some documents were sometimes marked Secret to avoid embarrassment to the government, Witness A said: "The answer to that question is No."

The trial was adjourned until Thursday.