Families of soldiers killed during the Iraq War have threatened legal action if a publication date for the Chilcot Inquiry report is not set within two weeks.
The Daily Mail said a group of at least 29 families issued the ultimatum to inquiry chairman Sir John Chilcot in a legal letter threatening to seek judicial review.
Prime Minister David Cameron has demanded a timetable for publishing his long-awaited report be set out "pretty soon", in a sign last week of his growing frustration at the delays in the process.
Sir John insisted last month that his inquiry - launched in 2009 - was making "significant progress", although he could not set a date of the publication of his findings.
He said officials were continuing to work through the so-called "Maxwellisation" process of assessing responses from individuals facing possible criticism in the final report.
But Roger Bacon, whose son Major Matthew Bacon, 34, died in a bomb blast in Basra 10 years ago, told the Daily Mail the delays were "morally reprehensible".
"We have lost our sons, daughters, brothers and sisters and if we don;t get answers as to why they died, it will all have been a waste of time," he said.
"It is utterly incomprehensible that the inquiry has been going on for six years and it is still not finished."
Former SAS commander General Sir Michael Rose, writing in the paper, added: "If Sir John Chilcot does not comply with the wishes of the families, then a judicial review into what might in the future become known as the 'Chilcot process' must take place."
While, the Prime Minister called for a timetable for publication to be set out as soon as possible, Whitehall sources do not expect this to happen before Parliament returns in September.