The Independent reported.
According to senior sources in the inquiry, the Chilcot team will now for the first time “challenge previous accounts of what happened” in the run-in to the war.
The inquiry team, however, believes "issues over declassification of some documents, and what evidence can be formally identified in the report, do remain."
The inquiry is expected to issue its report this autumn despite long delays.
The US and Britain invaded Iraq in blatant violation of international law in 2003 under the pretext of finding weapons of mass destruction (WMD) allegedly stockpiled by former Iraqi dictator Saddam Hussein. No WMDs, however, were ever discovered in Iraq.
More than one million Iraqis were killed as a result of the invasion and subsequent occupation of the country, according to the California-based investigative organization Project Censored.
Chilcot report into British involvement in the Iraq war and its aftermath will draw on unprecedented access to secret conversations between former British Prime Minister Tony Blair and former US president George W Bush.
A decade after Britain's controversial role in the US-led invasion of Iraq, the Chilcot inquiry, led by Sir John Chilcot, has finally been shown a fuller account of communications between Blair and Bush,