Families of soldiers killed in Iraq have threatened to mount legal action against John Chilcot if a publication date for his inquiry into the controversial conflict is not announced within two weeks.

The unprecedented step by 29 families was set out in a legal letter threatening to seek judicial review over the six-year inquiry into the 2003 Iraq war.

Sir John claims the inquiry, which has cost the taxpayer an estimated £10 million, has been drawn out to allow leading figures – such as then PM Tony Blair – the chance to respond to its findings.

However, the soldiers’ families have claimed he is acting unlawfully by consistently refusing to set a definitive publication date.

Roger Bacon, whose son Major Matthew Bacon died in Basra ten years ago aged 34, told the Daily Mail it was “morally reprehensible to keep delaying the publication of the report.”

“It is utterly incomprehensible that the inquiry has been going on for six years and it is still not finished,” Mr Bacon said.

The Iraq invasion, led by the US under George W Bush, saw 179 British soldiers killed. Estimates for the number of Iraqis (both civilans and soldiers) vary wildly, but are believed to be over 100,000.

The letter comes as David Cameron last week urged Sir John to publish his findings soon, telling reporters that he was “"fast losing patience" with the process.

Sir John insisted last month that the inquiry, launched in 2009 and which last held hearings in 2011, was making “significant progress”.

He claims officials are working through the “Maxwellisation” process of assessing responses from individuals possibly criticised in his report.

But Sir Jeremy Heywood, head of the civil service, told the BBC that the inquiry had repeatedly refused offers of additional help.

Families of soldiers have rejected Sir John's reasons for delaying the publication of his findings. A lawyer representing their case telling the BBC they were living under a “black cloud”.

"This suffering has only been compounded over time," said Matthew Jury, one of the solicitors representing the families.