The Iraq inquiry will round on former prime minister Tony Blair for telling Parliament that intelligence suggesting Saddam Hussein had weapons of mass destruction was "beyond doubt", according to the Mail on Sunday.
The Chilcot Inquiry will also criticise Mr Blair for failing to admit to a "secret pledge" with former US president George Bush in 2002 that he would go to war, the newspaper claimed.
In a report out this autumn, Mr Blair will also come under fire for operating a "sofa government" - a small core of key allies that confer away from formal meetings - so cabinet ministers were unaware of vital information.
The failure to develop robust post-war plans for Iraq will also be seized upon by the report. One MP said Mr Blair's "tapestry of deceit" over the Iraq war is now "unravelling".
The SNP's Westminster defence spokesman, Angus Robertson, also warned the report would raise "difficult questions" for MPs who backed the invasion.
Officials are currently writing the report and all witnesses will be given the chance to respond to any inaccuracies. Mr Blair's successor, Gordon Brown, set up the Chilcot Inquiry after criticism of previous probes.
Mr Blair mounted a vigorous defence of the 2003 invasion of Iraq when he appeared before the inquiry - insisting he had no regrets over removing Saddam and would do the same again - but said he "deeply and profoundly" regretted the loss of life.
A spokeswoman for the Chilcot Inquiry said: "We will not provide a running commentary on the inquiry."
A spokesman for Mr Blair said: "This is a deliberate attempt to pre-judge a report that hasn't even been written yet."