The inquiry was launched in July 2009 to look into the way decisions were made under the premiership of Tony Blair, to establish what happened and identify lessons that could be learned.
Chaired by Sir John Chilcot, its last public hearing took place on February 2, 2011, but it is yet to publish its report, more than 10 years since the start of the war which claimed the lives of more than 179 UK personnel and at least 100,000 Iraqis.
The delay has previously been ascribed to the scale of the report. The inquiry has dismissed claims the report is being delayed because the Government will not declassify Cabinet records of conversations between Tony Blair and former US president George W Bush.
Lindsey German of the Stop the War Coalition, said: "While the truth is being denied to those who suffered as a result of the war, Tony Blair sees fit to pronounce on a new war, this time with Syria. This totally misnamed envoy for peace in the Middle East should be in the dock."
Kate Hudson, of the Campaign for Nuclear Disarmament, said: "It is a grim irony news of the inquiry's delay has been drowned out by the drumbeats of a new war in the Middle East. Now more than ever, the delay of the Chilcot Inquiry is a gross abdication of responsibility."