The family of Denis Donaldson has accused the Irish government of “hypocrisy” over their treatment following the latest adjournment of the former Sinn Féin worker’s inquest.
It came after a hearing in Co Donegal was adjourned last week for the 26th time.
Mr Donaldson was shot dead at a remote cottage near Glenties in Co Donegal in 2006, months after it emerged he had been an MI5 agent for 20 years.
The 55-year-old was a former director of elections for Sinn Féin, and a close friend of hunger striker Bobby Sands.
His family has also criticised Gardaí for its handling of the case.
To date, no-one has been charged in connection with his murder.
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In a statement issued through their solicitor, they said they have been treated with “institutional contempt” .
“Now, state agencies in the south of Ireland appear to be acting with the same disregard for the rights of our family, most recently with the 26th adjournment of the inquest,” the family’s statement said.
“The Irish State and others stand in the dock of public hypocrisy if they continue to criticise the UK government’s approach to legacy while turning a blind eye to the mishandling of the case and the injustices which our family is facing.
“As a family we shall not accept this.”
They said Mr Donaldson being labelled an “informer” meant he had been “deemed in death as unworthy of any basic dignity”.
“National Security does not justify this,” they said of their situation.
“All we are asking is that statutory agencies, and those in authority or oversight, do their job in accordance with human rights and the rule of law.”
Last year the Police Ombudsman, Marie Anderson, said she believed there had been a “corporate failing” by the PSNI before Mr Donaldson’s murder over their assessment of his safety after the Sunday World newspaper published a picture of him at the rural cottage near Glenties where he was shot.
A Gardaí spokesperson said the service “does not comment on on-going investigations”.
“An Garda Síochána continues to appeal to anyone with information in relation to the murder of Mr Donaldson to contact us to help us bring those responsible to justice,” they said, adding the force had “made a number of arrests as part of its investigation” and “continues to liaise with our colleagues in the PSNI on this investigation”.