The family of Sinn Féin official and republican informer Denis Donaldson is to take legal action against An Garda Síochána (Irish police) in the European Court in Strasbourg.
They claim the gardai failed to properly investigate his killing in a remote cottage in County Donegal, and allege that PSNI officers may have contributed to his death.
Denis Donaldson was shot dead in April 2006, months after being exposed as an agent who had worked for the police and MI5 for 20 years.
The murder was claimed by the dissident republican Real IRA.
An inquest into his death was adjourned for the 17th time on Wednesday, after the gardai requested more time to pursue their enquiries.
The Donaldson family has alleged that his PSNI handlers may have been responsible for exposing his role as an informer.
They claim the gardai has breached Article 2 of the European Convention on Human Rights, which says there should be fully independent investigations of killings by the state.
The family also says repeated gardai requests for more time have denied them their legal right to have an inquest into the circumstances of the death.
The family solicitor told the BBC that legal action is now planned.
“The family will be consulting with their full legal team during the next few weeks with a view to lodging proceedings with the European Court, challenging the fact that they have not had an Article 2 compliant inquest or investigation into Denis Donaldson’s death, as is their right,” he said.
“This is the 17th time the gardai have made an adjournment application. They say it’s because they have to follow lines of inquiry, but we don’t know what those lines of inquiry are, we haven’t been advised exactly what they may be.
“The family are simply at their wits end. In the absence of any satisfaction from the Coroner’s Court, the director of public prosecutions or the gardai, they have been left with no alternative but to go to Strasbourg.”
The solicitor and the Donaldson family boycotted Wednesday’s inquest hearing in Donegal.
In a letter to the coroner, Ciaran Shiels of Madden & Finucane said they opposed the request for additional time, but viewed attending the hearing as a “pointless exercise”.