Families of British troops killed following the 2003 US-led invasion of Iraq have threatened to take a legal action unless a long-delayed inquiry into the war publishes its report by the year's end.

Lawyers representing 29 families say they would move to the London High Court if the Chilcot inquiry fails to give a publication deadline in two weeks. "There have been outrageous delays to date and it seems as though those delays would simply be interminable," Matthew Jury, a lawyer representing the families said.

Tony Blair giving evidence to the Iraq war inquiry (File photo)

The inquiry which was set up by former British Prime Minister Gordon Brown in 2009 and led by former civil servant John Chilcot initially expected to submit its report within a year. But, Chilcot has, time and again, cited complexities involved in the process.

The five-member panel held its last hearings in 2011. It is looking into the decisions made in the run-up to the start of the Iraq war and how the then Prime Minister Tony Blair decided to support the US-led invasion.

A total of 179 British soldiers were killed in Iraq between 2003 and 2010.

Dead British soldiers being airlifted from Iraq (File photo)

The Chilcot inquiry has given chances to many individuals including Blair to respond to criticism of them in the report. However, the delay has caused tensions both at political level as well as among families who lost their loved.

In June, Prime Minister David Cameron criticized the delay to the probe and said he wanted to see the real timetable for the final report. In response to a letter from Chilcot, Cameron said he is "fast losing patience" with the inquiry.

Chilcot insisted last month that his probe is making "significant progress" but could not give a timetable for publication.

Anti-war activists hold rally against delay in the Iraq war inquiry (File photo)

Some bereaved families suspect the probe is being skewed in favor of establishment figures.

"What Sir John doesn't understand is the strength of feeling amongst the bereaved. We want closure on this, it has to be done fair, it has to be done right but he's had time enough now," said Reg Keys who lost his son in Iraq in 2003.

Blair has been at the center of controversy with many accuse him of dragging Britain into the war under false pretexts. He has also faced criticism from his own party colleagues.

The front-runner Labour Party leadership candidate, Jeremy Corbyn said he believed the conflict was illegal and Blair should be tried for war crimes if it was shown he had broken international law.