George Galloway says Bashar Assad is "bad enough" to have gassed his own people - but questioned whether he was "mad enough".
The controversial Respect MP said that to have launched the action just before the arrival of UN inspectors would have been "the definition of madness".
Galloway cited just a survey that found just 11% of people would support action, and said the Syrian rebels were persecuting all religious minorities in the country.
"Everybody knows they're bad enough to do it, the question is are they mad enough to do it?," he told MPs.
"To launch a chemical weapons attack in Damascus on the very day that a United Chemical chemical weapons inspection team arrived in Damascus, must be a new definition of madness.
"And of course, if he is that mad, how mad is he going to be once we have launched a blizzard of Tomahawk cruise missiles on to his country?"
People were cautious about intervening because they could "see the character of the Syrian rebels", Galloway claimed, citing reports that a rebel commander ate the heart of an opponent.
He was also challenged by Tory MP Matthew Offord over his reported comments that Israel had given chemical weapons to the rebels.
"I said no such thing," he replied.