Boris Johnson has called for a second vote on intervention in Syria, saying those behind the chemical attack in Damascus should be "punished".
Over the weekend, George Osborne and William Hague all but ruled out returning to Parliament to seek authorisation for military action, and on Monday morning Nick Clegg said he "can't foresee any circumstances" in which it could happen.
But the Mayor of London said that if UN inspectors found new evidence, he saw "no reason" why a new motion could be put before MPs, and branded Ed Miliband "weaselly" in his response.
Writing in the Telegraph, he said: "The UN will have had longer to report.
"If there is new and better evidence that inculpates Assad, I see no reason why the Government should not lay a new motion before Parliament, inviting British participation – and then it is Ed Miliband, not David Cameron, who will face embarrassment."
A gas attack was "a crime that we cannot allow to go unpunished," Johnson wrote.
Asking what a strike should entail, he said: "Is this a slap on the wrist, or six of the best? Or is it regime change?"
The government motion on intervention was defeated on Thursday by a combination of Labour and backbenchers fearful of a repeat of the Iraq fiasco.
But Johnson said that if Tories and "Blairites" combined, they would have the numbers to agree "a calibrated and limited response to a grotesque war crime."