Mr Donaldson, formerly a senior member of Sinn Féin, was shot in April 2006, less than six months after being publicly named as a British intelligence agent.

The paramilitary group known as the Real IRA, or latterly the New IRA, claimed responsibility for the killing.

In a statement issued on behalf of Mr Donaldson’s family, Ciarán Shiels of Madden & Finucane Solicitors said: “Today marks ten years since Denis was murdered. No-one has yet been held to account for either his exposure as an agent or his subsequent murder. An inquest has still not even been held, yet it has been adjourned on 18 occasions by the Coroner upon the application of An Garda Siochana.

“For the final time, the Donaldson Family are calling upon An Garda Siochana to fully cooperate with the ongoing Police Ombudsman’s investigation of Denis’ exposure in the North by giving the investigators unfettered access to his private journal which was being written by him at the time of his murder. The Office of the Police Ombudsman for Northern Ireland is a fellow Law Enforcement Agency and is following a definite line of enquiry in relation to the Family’s complaint. Despite repeated requests, they have been refused access to this document.

“The Donaldson Family are appealing to all relevant State Agencies, North and South, to finally remove any further obstructions to the truth and sincerely acknowledge the terrible damage they caused, or continue to cause, to the family by their actions and omissions. The onus is with them to remedy these wrongs.”