London:
Any action against Damascus would be designed to deter the use of chemical weapons not just in Syria but in the rest of the world, British Prime Minister David Cameron said on Tuesday.
After announcing that the British parliament would be recalled on Thursday to debate possible action, Cameron described the use of chemical weapons as "morally indefensible".
But any military action against Syria would have to be proportionate and legal, Mr Cameron said.
The British PM said no decisions had been taken but Britain and its allies had to consider whether targeted military action was required to "deter and degrade the future use of chemical weapons".
"This is not about wars in the Middle East; this is not even about the Syrian conflict. It's about the use of chemical weapons and making sure as a world we deter their use," he said.
"The question we need to ask is whether acting or not acting will make the use of chemical weapons more prevalent?"
He stressed that "any decision would have to be proportionate, would have to be legal, would have to be about specifically deterring the use of chemical weapons".
He said the regime of Syrian President Bashar al-Assad had the stockpiles, the opportunity and the motive to use chemical weapons, whereas the chances of the opposition having used them was "vanishingly small".
Mr Cameron's spokesman confirmed on Tuesday that Britain's armed forces were drawing up contingency plans for military action in Syria.
Britain's Deputy Prime Minister Nick Clegg said "what we're considering is a serious response" to chemical weapons use.
"What we're not considering is regime change, trying to topple the Assad regime, trying to settle the civil war in Syria one way or another."
"We're not considering an open-ended military intervention with boots on the ground like we saw in Iraq."
He said that killing with chemical weapons was a "repugnant crime" and a "flagrant abuse of international law".
It would set a "very dangerous precedent" if "brutal dictators" felt they could get away with using these "heinous weapons with impunity".
Britain, France and the United States have all said they believe the alleged gas attacks that killed hundreds of civilians near Damascus last week were launched by the regime.
US Defence Secretary Chuck Hagel said US forces were ready to launch strikes against the Syrian regime once President Barack Obama gave the order.
In Paris, French President Francois Hollande said his country was "ready to punish" those behind the alleged attacks.
British lawmakers will debate Syria on Thursday.
In tense scenes a decade ago, the House of Commons gave its approval to the invasion of Iraq by a comfortable margin in March 2003 when Tony Blair was prime minister.
Two years ago, the Commons gave overwhelming support - 557 to 13 - to airstrikes to assist the Libyan rebel forces.
It is the fourth time Cameron has recalled parliament during a recess - previously a rarely-used step. The last time was for a day of tributes to former PM Margaret Thatcher, who died in April.