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, the prime minister said.
Cameron 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".
Cameron's spokesperson 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 prime minister Margaret Thatcher, who died in April.
COMMENTS (7)
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@Really?: "Really? What punishment was given to Israel when they used phosphorus in their attacks on Palestine during 2009?" . Like it or not. One of the basic rules of the game is that the rules are rarely applied evenly.
@whitesky:
Chemical weapons are far more indiscriminate than conventional weapons and results are more devastating - Syria is case in point with over 3,000 admitted to hospitals. Chemical weapons are nasty - hence the reason they are classified as Weapons of Mass Destruction.
Can any UK PM speak his/her own words with his/her tongue, instead of reading out the script handed over by USA President? However, same sort allegation were leveled against Iraq in 2003 to make the ground for attack, so what happened to those? Any guesses?
Really? What punishment was given to Israel when they used phosphorus in their attacks on Palestine during 2009?
MR. Kamron, Don't try to entangle yourself into middle east politics. Let them to fight and decide their future.Your imperialist policy would one day reverse at your own country when the people of middle east asia and africa would meddle in your affairs.
What is the difference in bombing of a high power explosion and use of chemical weapon. Both are meant to kill the humans / society( bombing destroy the property too). May be it the variance of degree of power to kill / destroy.. if forces deployed ( other than chemical weapons) is tolerable to all to keep mum / non intervention why the limited use of chemical weapons can be an excuse to intervene / attack by West / USA situated thousands of miles away (unaffected physically) . Sir, Everything is justified in war from both sides and Syrian scenario is not less than a war/ civil war. Personally i an against the use of chemical weapon
So you would go in and kill another 1000 innocents to punish Basharulasad? Wow! What a plan! Clap clap!