Any intervention in Syria would not be about the conflict itself but preventing the use of chemical weapons by any regime, David Cameron said, stressing no decisions about British involvement had been made.
The US has said there is “clear” evidence that President Bashar al-Assad’s government was behind last week’s attack on the outskirts of Damascus but Russia, a key ally of Syria, has questioned this.
Decisions about potential future British involvement have not been taken, Mr Cameron said, adding Parliament, which has been recalled, was the “right place to set out all of the arguments”.
He said action must be “proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons”.
Mr Cameron said any action had to be legal, proportionate and a deterrent to the future use of such weapons.
He added: “Let me stress to people, this is not about getting involved in a Middle Eastern war or changing our stance in Syria, or going further into that conflict. It’s about chemical weapons. Their use is wrong and the world should not stand idly by.”
Mr Cameron said the question for Britain is whether failing to act this time would lead to more use of chemical weapons in Syria and elsewhere in future.
“It must be right to have some rules in our world and try to enforce those rules,” he said.
Mr Cameron said tomorrow’s debate would ensure “proper” scrutiny and allow the Government to listen to MPs.
He said: “Obviously this is a developing situation, as I say, decisions have not been taken, but we shouldn’t stand by when we see this massive use of chemical weapons and appalling levels of suffering.
“I think in Parliament is the right place to set out all of the arguments to all the questions.
“But I would say this to people - there is never 100 per cent certainty, there is never one piece or several pieces of intelligence that give you absolute certainty.
“But what we know is this regime has huge stocks of chemical weapons.
“We know they have used them on at least 10 occasions prior to this last widescale use.
“We know they have both the motive and the opportunity whereas the opposition does not have those things and the opposition’s chance of having used chemical weapons in our view is vanishingly small.”