MPs should reconsider the possibility of UK military action against Bashar Assad if the United States decides to launch strikes on Syria, Boris Johnson has said.
The London Mayor said he believed US politicians would back Barack Obama over the use of force and Parliament should then "think again" about committing British personnel to an international alliance.
Downing Street has played down the chances of a fresh bid to gain the support of MPs for military action, but Mr Johnson said it would be appropriate to look at the new evidence and the US plan for action.
Asked what could be achieved by bombing Syria, Mr Johnson said: "It will show that in the end, when a tyrant decides to use unlawful weapons, gas, to murder innocent civilians, we will not stand idly by."
He told LBC 97.3: "I have been very hostile and sceptical about British involvement in Syria, we all remember what happened in Iraq. But if you see someone gassing innocent civilians, that is a chilling, chilling memory for people in Europe of what happened in the 1930s and 1940s and of the cost of doing nothing."
Mr Johnson added: "What I have said is if it can be proved that it really was the Assad regime that was responsible, and there's more proof, there's more evidence coming in, and if the Americans - who after all are the only real power that has the men and the material to deliver any retribution - can come up with a plan that is coherent, that is limited, that is punitive in some way but does not actually engage the West in some long-term commitment in a Syrian conflict, then I think there is a case for Britain looking again at whether or not we should participate."
If President Obama secured the backing of US politicians, Mr Johnson said "in those circumstances I think it would be appropriate for us to look at what new evidence there is, look at what the Americans are actually proposing, see whether it's coherent, see whether it's limited, see whether it's commensurate with what has happened, see whether it will deliver a plausible outcome".
He added: "I think it would be reasonable in those circumstances for Parliament to think again."
The Commons defeat was a significant blow for David Cameron but Mr Johnson praised the way the Prime Minister had highlighted the Syria issue.
He said: "I think David Cameron has been in the lead on this from the beginning. I think he was absolutely crucial in getting President Obama to focus on this.
"I think as soon as it became clear that the British Parliament had reservations, the importance of the British Parliament was underscored by the decision of Barack Obama immediately to reverse ferret, as we say, to say 'I'm going to put it to Congress and to have a longer period of examination and reflection'.
"Many people in this country will think that's the right way forward: let's look at the evidence in more detail, let's see if we can come up with a really coherent and a thought out approach."
The Mayor also attacked the "absolutely catastrophic" performance of Labour leader Ed Miliband, who sparked fury in Downing Street by refusing to back the Government's motion last week.
Mr Johnson said: "He is going around claiming he is the victor of the do nothing party but actually he is simultaneously saying he is prepared to do something. Which is it? He needs to make up his mind."
Mr Johnson compared Assad to Adolf Hitler and Saddam Hussein, and said that the world should not repeat its failure to deal with them at an early stage.
Acknowledging that the West did not punish Saddam for his use of chemical weapons against civilians in Halabja in 1988, Mr Johnson told LBC: "Just because you do the wrong thing once, it doesn't mean you should do the wrong thing again and again.
"We did the wrong thing in the case of Hitler. We didn't stand up to him early enough.
"Yes, it's true that we failed to punish Saddam for what he did at Halabja. But just because you get it wrong once doesn't mean you should get it wrong again."
Mr Johnson added: "I have very, very strong doubts about us getting involved in any way in trying to change the outcome of the Syrian conflict, trying to get rid of Assad.
"We don't know who's going to replace Assad. We don't have a clue who these guys are in the Free Syrian Army really. To what extent is there penetration by al Qaida? All these questions we don't have the answer to.
"But if someone uses gas to murder large numbers of innocent men, women and children, it is very difficult for a civilised country like ours - a great country that has a history of sticking up for oppressed people around the world - it is very , very difficult for us just to do nothing."
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