We know that Blair had already decided in 2002 to join the US in a war to topple the regime in Iraq, principally on account of his ideological identification with Bush’s neo-conservative imperial agenda. Blair’s problem was simple: this reason would never wash in Britain. He therefore decided to tell lies.
Blair lied in telling the country that Saddam's refusal to declare and give up his possession of WMDs was the casus belli. This was a lie because Blair had already made the decision to go to war to bring about regime change irrespective of whether Saddam had WMDs.
To support that lie Blair told another lie; namely that he had credible evidence that Iraq possessed WMDs. He had no such evidence; and Iraq possessed no WMDs.
Blair made use of the crown prerogative to go to war. He justified that on false pretences and he took Britain to war without legal justification in international law.
What does all that make Blair?