Former MI5 officer David Shayler arrives at the Bloody Sunday Inquiry

An IRA agent who alleged Sinn Fein's Martin McGuinness told him he fired the first shot on Bloody Sunday was not reliable, a former MI5 officer has said.

David Shayler, 37, told the Saville Inquiry that the informer, codenamed Infliction, had supplied intelligence to MI5 that was inaccurate.

The Saville Inquiry is examining the events of 30 January 1972 when 13 civilians were shot dead by British army soldiers during a civil rights march in Londonderry. A 14th person died later.

MI5 has said Infliction was a leading member of the Provisional IRA and was close to Martin McGuinness.

Mr Shayler told Thursday's sitting he first came across Infliction in 1993 while he was dealing with another target and went to a source in Section T8, which ran agents, to seek an assessment of his reliability.

He said the person he spoke to told him the agent was unreliable.

"I asked for more details which I could refer to in my assessment of my target.

"He told me that Infliction had at one time been totally believed and was regarded as reliable.

"Then there was a case where they had initially gone with Infliction, when his information was contradicted by another source and it was found that the other source had been accurate."

Judgement questioned

MI5 have challenged this evidence, with one of Infliction's handlers telling the tribunal on Wednesday that the informer was generally honest and reliable.

The handler said Mr Shayler did not know Infliction's identity and would not have had enough information to make any sort of judgment about his reliability.

David Shayler was sentenced to six months in jail last year after being found in breach of the Official Secrets Act.

The inquiry, which usually sits at the Guildhall in Derry, is currently hearing evidence from military witnesses and others in London because of concerns for their safety.

Lord Saville of Newdigate and the Commonwealth judges accompanying him on the Bloody Sunday Inquiry began their work nearly four years ago.

They are not expected to report back until 2004.

The Bloody Sunday Inquiry was established in 1998 by Prime Minister Tony Blair after a campaign by families of those killed and injured.

They felt that the Widgery Inquiry, held shortly after the shootings, did not find out the truth about what happened on Bloody Sunday.