Du Plantier was beaten to death at her holiday home in Schull, Co Cork, on 22-23 December 1996. Nobody has ever been charged in relation to the murder.

Bailey was targeted by gardai as the chief suspect in the killing. However, one of the gardai's major witnesses, whose evidence linked Bailey to the scene of the crime, dramatically withdrew her statements last year.

In papers being prepared for the lawsuit, Bailey's legal team will claim that he was deliberately singled out by investigating gardai and that witnesses were coerced to support the contention that he killed du Plantier. He will also claim that he suffered alleged physical and psychological mistreatment by gardai during the initial investigation.

When contacted yesterday by the Sunday Tribune, Bailey's solicitor Frank Buttimer said: "I can confirm that my client, Ian Bailey, will be initiating a civil action against the state and other parties."

Legal papers in the case will be filed in the action in "coming weeks", Buttimer said. The Sunday Tribune understands that Jules Thomas, Bailey's long-term girlfriend, is to initiate separate action against gardai and the state.

It is also understood that one of the key witnesses who will be called at Bailey's civil action will be local shop owner Marie Farrell.

Farrell had given statements to gardai in which she claimed that she was driving her car in the early hours of 23 December 1996 at Kealfada Bridge near du Plantier's home when she saw Ian Bailey walking along the road. Bailey repeatedly denied that he was near the murder scene on the night du Plantier was murdered.

However, Farrell has since claimed that her initial statement was incorrect and groundless. She said that she was conducting an affair with a man on the night in question and she feared that if she did not cooperate with gardai, members of the force would reveal details of the extra-marital relationship to her husband.

In the aftermath of Farrell's retraction of her initial statement, Garda Commissioner Noel Conroy ordered an immediate internal inquiry into the initial garda investigation of du Plantier's killing. This is currently underway.

Conor Brady, of the Garda Ombudsman Commission, told the Sunday Tribune that a retrospective probe into the garda handling of the case could come under the commission's remit when it becomes operational next spring. A spokesman for the Department of Justice declined to comment when contacted as "the legal papers have not yet been filed".