Intelligence officers have disclosed that just the day before Mr Blair went to visit president George Bush in April 2002, he appeared to accept this but returned a "changed man" and subsequently ordered the production of dossiers to "find the intelligence" that he wanted to use to justify going to war.
This and other secret evidence (given in camera) to the inquiry will, The Independent on Sunday understands, be used as the basis for severe criticism of the former prime minister when the Chilcot report is published.
Mr Blair is said to have "realised" and "understood" that Libya was the real threat and that he knew "it would not be sensible to lead the argument on Saddam and the WMD issue" according to evidence of a conversation on 4 April 2002, the day before he flew to the US to spend a weekend with Mr Bush.
This was disclosed in a closed evidence session with one of MI6's most senior officers, named as SIS4. Although details have been redacted, the transcript, later released online with little fanfare, states that Mr Blair "realised that the WMD threat from Libya was more serious than from Iraq".
During a closed session with former MI6 chief Sir Richard Dearlove, redacted evidence claims Mr Blair "had understood that Libya posed a bigger threat than Iraq, and understood the risk, therefore, of focusing on WMD in relation to Iraq". It refers to a meeting held by Mr Blair at Chequers days before the visit to Mr Bush's ranch in Crawford, Texas, but is unclear whether the claims were made by Sir Richard or another individual. What is clear is that in 2002, British intelligence "discovered that Libya has an active nuclear weapons programme", according to Sir Richard.