Families of British soldiers killed in Iraq are threatening to take legal action against Sir John Chilcot, who led the inquiry into the conflict.
He has said publication of his report has been delayed to allow those who have been criticised to respond.
Lawyers for the soldiers’ families claim he acted unlawfully by refusing to set a deadline for publication.
The five-member panel, led by Sir John, began its work in 2009 and held its last hearings in 2011.
The inquiry took evidence from hundreds of witnesses, including former Prime Minister Tony Blair, and is set to cost taxpayers £10m.
The delays, to allow those who might face criticism in the report to argue their case, have led to frustration among some of the soldiers’ families.
Lawyers acting for 29 of them have written to Sir John calling for him to set a deadline for witnesses to respond and to promise the report will be published by the end of the year or they will take their case to the High Court.
Reg Keys, whose son L/Cpl Thomas Keys was killed in Iraq in 2003, said he was “losing patience” and the families had come to the “end of our tether”.
“It’s got to the stage now where Iraq is like an open wound and it is continually prodded and opened with all these delays,” he told the BBC.
“The families want closure now, we need to know why our loved ones died.
“Those who were seriously wounded and maimed, those families will want to know why their loved ones have ended up in that state.”
Matthew Jury, one of the solicitors representing the families, said the long delay has caused them further pain.
“This suffering has only been compounded over time,” he said.
“They describe it to me as a black cloud hanging over their heads and the only way to disperse that cloud, for them to get some degree of closure, is for this report to be published and for them to finally know the truth.”
David Cameron recently said he was “fast losing patience” with the process and wanted to see a timetable for the publication of the report.
Head of the civil service Sir Jeremy Heywood said the inquiry had repeatedly turned down offers of extra assistance to help speed up the report.
Sir John has always declined to say exactly when the report would be released but has said he wholeheartedly shares the wish for it be made available as soon as possible.