Why has the report taken so long?
The inquiry was announced by the former prime minister Gordon Brown in June 2009. The assumption, at the time, was that it would take two or three years at most but it soon became apparent it would be much longer.
The first phase was a series of public hearings in which senior politicians, military commanders and diplomats were questioned. This was briefly interrupted by the 2010 general election but came to an end in February 2011.
The subsequent delays which followed have angered the relatives of those killed as well as politicians on all sides of the debate.
The report is a massive undertaking, almost unprecedented in scope, and the inquiry conceded that it underestimated the scale of the task and needed more resources to begin with. There have also been unforeseen events such as Sir Martin Gilbert's death.
But the main reason for the delay was the long tussle between the inquiry and the government over which classified material could be published alongside the report, or referred to in it.