Watch our explainer on what it is and what it will cover:
WHAT IS THE CHILCOT INQUIRY?
Officially called the Iraq Inquiry, it's an investigation by a committee of Privy Counsellors into the UK's role in the controversial Iraq War.
It is chaired by Sir John Chilcot.
It will look at events in the run-up from 2001, the military action from 2003, and the aftermath.
WAS THE PUBLIC MISLED?
The 2003 invasion sparked much criticism of Prime Minister Tony Blair, and also his spin doctor Alistair Campbell. The report will deal with the controversial issue of whether the public was misled. There were accusations of exaggerating the case for war- but that's something they've always denied:
years to complete
words make up the report
So far it has cost £10,375,000 of public money.