Tony Blair's "unhinged" attempt to rewrite history is undermining arguments for Western intervention in Iraq, Boris Johnson has claimed, in an extraordinary personal attack on the former prime minister.
Mr Blair took to the media to make the case for a tough response to the extremist insurgency in Iraq - insisting it was caused by a failure to deal with the Syria crisis, not the 2003 US-led invasion.
His intervention was met with widespread criticism from Labour figures and others as extremists posted pictures apparently showing the massacre of dozens of Iraqi soldiers by jihadist fighters.
Amid international outrage at the atrocity, US President Barack Obama is weighing up what help to give Baghdad to counter the land-grab by the al-Qaida-inspired Islamic State of Iraq and the Levant (Isis).
The Pentagon has sent an aircraft carrier to the Gulf in advance of potential air-strikes amid calls on Mr Obama to talk with Iran over a co-ordinated response.
After talks with Iraqi foreign minister Hoshyar Zebari, Foreign Secretary William Hague repeated calls for the country's divided political leaders to present a united front against the Isis threat.
In the strongest condemnation of Mr Blair, the London Mayor went as far as accusing the ex-Labour leader of having sent British forces into the bloody conflict in part to gain personal "grandeur".
In his Daily Telegraph column, Mr Johnson said Mr Blair and then-US president George W Bush had shown "unbelievable arrogance" to believe toppling Saddam Hussein would not result in instability which resulted directly to the deaths of 100,000 Iraqis and hundreds of British and American troops
He suggested there were "specific and targeted" actions that could be taken by the US and its allies to deal with latest threat - as Barack Obama considers a range of military options short of ground troops.
But he said that by refusing to accept that the 2003 war was "a tragic mistake", " Blair is now undermining the very cause he advocates: the possibility of serious and effective intervention.
"Somebody needs to get on to Tony Blair and tell him to put a sock in it, or at least to accept the reality of the disaster he helped to engender. Then he might be worth hearing," Mr Johnson concluded.
The row over the events of 11 years ago came amid suggestions of serious atrocities being committed in the militants' advance.
Taking on critics in an eight-page essay on his website, Mr Blair rejected as "bizarre" claims that Iraq might be more stable today if he had not helped topple Saddam.
The former premier - now a Middle East peace envoy - said Iraq was "in mortal danger", but pinned the blame on the sectarianism of the al-Maliki government and the spread of Syria's brutal three-year civil war.
"The choices are all pretty ugly, it is true," he wrote in a push for military intervention - though not necessarily a return to ground forces.
"But for three years we have watched Syria descend into the abyss and as it is going down, it is slowly but surely wrapping its cords around us, pulling us down with it.
"We have to put aside the differences of the past and act now to save the future. Where the extremists are fighting, they have to be countered hard, with force.
"Every time we put off action, the action we will be forced to take will be ultimately greater."
Former foreign minister Lord Malloch Brown urged Mr Blair to "stay quiet" because his presence in the debate was driving people to oppose what might be the necessary response.;
Clare Short, who quit Mr Blair's cabinet in the aftermath of the 2003 invasion, said he had been "absolutely, consistently wrong, wrong, wrong" on the issue, and opposed more strikes.
Ukip leader Nigel Farage dismissed Mr Blair as an "embarrassment" who should hold his tongue - and demanded "an end to the era of military intervention abroad".
Scotland's First Minister Alex Salmond added his voice to the chorus, accusing Mr Blair of "breathtaking amnesia" over his reasons for invading Iraq.
And Sir Christopher Meyer, Britain's ambassador to the US from 1997 to 2003, said the handling of the campaign against Saddam was "perhaps the most significant reason" for today's violence.
"We are reaping what we sowed," he said.
Former Liberal Democrat leader Lord Ashdown said: "I'm having a bit of a difficulty getting my mind round the idea that a problem that has been caused or made worse by killing many many Arab Muslims in the Middle East is going to be made better by killing more with western weapons."
In his column, Mr Johnson wrote: " I have come to the conclusion that Tony Blair has finally gone mad.
"In discussing the disaster of modern Iraq he made assertions that are so jaw-droppingly and breathtakingly at variance with reality that he surely needs professional psychiatric help."
He went on: " There are more than 100,000 dead Iraqis who would be alive today if we had not gone in and created the conditions for such a conflict, to say nothing of the troops from America, Britain and other countries who have lost their lives in the shambles.
"That is the truth, and it is time Tony Blair accepted it.
"We utterly blitzed the power centres of Iraq with no credible plan for the next stage - and frankly, yes, I do blame Bush and Blair for their unbelievable arrogance in thinking it would work."
Backing the case for intervention, he said: "It would be wrong and self-defeating to conclude that because we were wrong over Iraq, we must always be wrong to try to make the world a better place.
"But we cannot make this case, for an active Britain that is engaged with the world, unless we are at least honest about our failures.
"It might be that there are specific and targeted things we could do - and, morally, perhaps should do," in both Iraq and Syria, he said.
In his pointed attack on Mr Blair's motivation, he wrote: " Blair went in fundamentally because he (rightly) thought it was in Britain's long-term interest to be closely allied with America, and also, alas, because he instinctively understood how war helps to magnify a politician.
"War gives leaders a grandeur they might not otherwise possess. If you hanker after Churchillian or Thatcherian charisma, there is nothing like a victorious war."