The former Prime minister insisted he acted “in good faith” and that the report should “lay to rest” allegations of lying.
But Scotland’s First Minister Nicola Sturgeon wrote: “#Chilcot damning. War not last resort, based on flawed, unchallenged intel and unsatisfactory legal decisions.”
Mrs Sturgeon said the revelation that Tony Blair had told George Bush “I’ll be with you whatever” in 2002, “suggests pre-determination and lack of proper decision making”.
And she described as “appalling” the lack of preparation for the aftermath of the conflict, and the failure to properly equip troops - both of which were findings of the report.
Green MP Caroline Lucas said the findings of the inquiry were “shocking”, tweeting: “#Chilcot nails lie that invasion was necessary. Diplomacy had not been exhausted.”
Footage posted on Twitter by Press Association reporter Ryan Hooper also showed Ms Lucas telling campaigners that the report “confirms all your worst fears”.
Conservative MP James Heappey, who was previously a serving officer in Iraq with 4th Battallion The Rifles, wrote: “So why was I really in Basra?”
Mr Blair his insisted his decision to commit British forces to the invasion of Iraq was taken “in good faith” but he would “take full responsibility for any mistakes”.
Mr Blair was severely criticised by Sir John Chilcot’s inquiry into the Iraq war, but the former premier said the report “should lay to rest allegations of bad faith, lies or deceit” against him.
He added that his decisions were taken “in what I believed to be the best interests of the country” and added that he still believed “it was better to remove Saddam Hussein”.
Sir John’s report said the circumstances in which Mr Blair and Attorney General Lord Goldsmith decided that there was a legal basis for UK military action in Iraq were “far from satisfactory”.
He also said there was “no imminent threat” from Saddam at the time of the invasion and the intelligence about his weapons of mass destruction was “flawed”.
But Mr Blair said the report found there was “no falsification or improper use of intelligence”, “no deception of Cabinet” and “no secret commitment to war” was given to US President George Bush.
He said the report “does not make a finding on the legal basis for military action but finds that the attorney general had concluded there was such a lawful basis” by March 13 2003.
Mr Blair said: “The report should lay to rest allegations of bad faith, lies or deceit. Whether people agree or disagree with my decision to take military action against Saddam Hussein; I took it in good faith and in what I believed to be the best interests of the country.”
He acknowledged the Chilcot report made “real and material criticisms” of “preparation, planning, process and of the relationship with the United States”.
Mr Blair, who will set out a full response to the report later, said: “I will take full responsibility for any mistakes without exception or excuse.
“I will at the same time say why, nonetheless, I believe that it was better to remove Saddam Hussein and why I do not believe this is the cause of the terrorism we see today whether in the Middle East or elsewhere in the world.
“Above all I will pay tribute to our armed forces. I will express my profound regret at the loss of life and the grief it has caused the families, and I will set out the lessons I believe future leaders can learn from my experience.”