David Cameron has said the alleged use of chemical weapons by the Syrian government is "morally indefensible" after he recalled Parliament to discuss responses to the crisis.
The prime minister said the world could "not stand idly by" in the face of the "massive use" of banned weapons.
But any military action would have to be proportionate and legal, he added.
The Syrian government said it was not responsible and the US and others were using it as an excuse to attack it.
The UK is considering military options following last week's suspected attack, which is being investigated by the United Nations.
Mr Cameron said he believed that the Syrian government had the "motive and the opportunity" to use chemical weapons while the likelihood of opposition forces being the perpetrators was "vanishingly small".
"What we have seen in Syria are appalling scenes of death and suffering because of the use of chemical weapons by the Assad regime," he said. "I don't believe we can let that stand."
While there was no question of the UK and its allies seeking to alter the outcome of the military struggle in Syria, they must decide whether limited military action was needed to "deter and degrade the future use of chemical weapons".
'Ready to go'
Downing Street confirmed that the prime minister had spoken to President Barack Obama on Tuesday evening but said no decisions would be taken before a meeting of the National Security Council (NSC) on Wednesday.
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.
US Defence Secretary Chuck Hagel said US forces were "ready to go" if given the order by President Obama but the facts of what had happened needed to be fully established before any decisions were taken.
A report on chemical weapons use being compiled by US intelligence would be published later this week, White House spokesman Jay Carney has said.
The Syrian authorities have blamed opposition fighters, with whom they have been involved in a civil war for more than two years.
UN weapons inspectors examined the scene of one of the alleged attacks on Monday - after being delayed by a sniper attack on their convoy - but on Tuesday postponed a second trip to rebel-held suburbs of Damascus until Wednesday because of safety fears.
After cutting short his holiday to deal with the crisis, the prime minister said the House of Commons Speaker, John Bercow, had granted his request for Parliament to be recalled from its summer recess four days early, and MPs would have the chance to vote on a "clear motion" of action.
Mr Cameron has held meetings with senior colleagues, including his deputy Nick Clegg and Foreign Secretary William Hague, ahead of a meeting of the NSC on Wednesday.
Mr Clegg, whose Liberal Democrats opposed the intervention in Iraq, said there would not be a "boots-on-the-ground invasion" of Syria.
He said: "The use of chemical weapons on men, women and children is a flagrant abuse of international law and if we stand idly by we set a very dangerous precedent."
He added that "any steps taken will have to be legal".
Labour leader Ed Miliband said there was a "lot of evidence" pointing to the past use of chemical weapons by the regime but any international response must be legally sound and be based on precise, achievable objectives.
It is understood the most likely military response to Wednesday's suspected chemical weapons attack would be a one-off or limited guided missile strikes on Syrian military targets fired from US Navy warships.
Thursday's Commons vote on the issue would not be legally binding but No 10 sources said the prime minister would listen to the will of Parliament amid concerns from MPs from all parties about the consequences of military intervention.
Although the Commons voted on UK military intervention in Iraq in 2003 and Libya in 2011, Mr Cameron has the final say on deploying troops in conflicts, using Royal Prerogative powers.
Conservative MP Richard Ottaway said many MPs felt they had been "misled" over Iraq and urged ministers to make any intelligence about the chemical attacks available to members of Parliament's Intelligence and Security Committee - which meets in private.
Meanwhile, General Lord Dannatt - until 2009 head of the British Army - said military action without UN backing would be "wrong", and called on the PM to "convince the British people that there is a clear case for intervention".
Moscow has warned that any foreign involvement in Syria without a UN mandate would be "a grave violation of international law".
The UN Security Council is made up of 15 states, including five permanent members - China, Russia, France, the US and the UK - who have the power to veto any resolution.
The Obama administration is reportedly studying the Nato-led military campaign against the Federal Republic of Yugoslavia in 1999 as a potential precedent for intervention without a specific UN mandate.
The US and UK supported more than 70 days of air strikes against the regime of Slobodan Milosevic - in the face of Russian opposition - to protect civilians from further attacks in Kosovo.
But Syria's foreign minister, Walim Moualem, said the US and its allies were using the alleged chemical attack as a pretext to intervene in the bitter conflict in the country and any "act of aggression" would strengthen the hand of radical elements linked to al-Qaeda.