Prime Minister David Cameron gathered the UK's armed forces and security chiefs with key cabinet ministers in Downing Street earlier for emergency talks over possible military action.
Chief of the Defence Staff General Sir Nick Houghton and MI6 chief Sir John Sawers, Defence Secretary Philip Hammond and Attorney General Dominic Grieve were among those around the Cabinet table at 10 Downing Street to discuss the options.
"The National Security Council met this afternoon to consider the Government's response to the appalling chemical weapons attack near Damascus last week," a Number 10 spokeswoman said.
"The NSC agreed unanimously on a recommendation that the Cabinet will consider tomorrow.
"Ministers agreed that the (Bashar) Assad regime was responsible for this attack and that the world shouldn't stand idly by and that any response should be legal, proportionate and specifically to protect civilians by deterring further chemical weapons use."
The NSC also backed the UK's move to table a draft resolution to be debated by the five permanent members of the UN Security Council later in an effort to show willing to secure international support.
The resolution would authorise "all necessary measures to protect civilians" and condemns "the chemical weapons attack by Assad".
"The NSC agreed unanimously that the use of chemical weapons by Assad was unacceptable - and the world should not stand by," the Prime Minister wrote on Twitter.
The meeting is expected to help shape the wording of a motion to be voted on tomorrow by MPs, who have been recalled from their summer break to debate the Syria crisis.
Mr Cameron insists any use of force would only be a response to the use of banned chemical weapons and would be legal and proportionate.
But there is widespread scepticism among MPs of all parties about the wisdom of a Western military intervention in the brutal civil war.
Labour has made a fresh effort to make UN backing for a military intervention one criterion for voting with the Government in tomorrow's vote.
Shadow foreign secretary Douglas Alexander said any proposed action "must have a clear legal basis" and warned that there must be direct involvement from the United Nations.
A senior Liberal Democrat source, however, conceded that the Government was not optimistic of securing Russian and Chinese support.
"We are not expecting a massive change of heart... but it is important to try," they said.
United Nations involvement had "always been at the forefront of the Government's mind", the source insisted, and was not a direct response to Labour's calls.
First Minister Alex Salmond said the case for military action in Syria had not been made.
"The Scottish Government condemns unreservedly the actions of the Assad regime over recent months and years," he said in a statement.
"In particular, we condemn and deplore any use of chemical weapons by any party as a crime against humanity. If the findings of UN inspectors do point to this appalling attack having been perpetrated by the Syrian regime, Assad and those responsible should face the full accountability of the International Criminal Court.
"Any resort to military action should always be approached carefully, on an evidential base, and within a clear legal framework - and only after full consideration of the aims, objectives and consequences. At this stage, we consider that these criteria have not been met and therefore that the case for military action in Syria - or the UK's participation in it - has not yet been made.
"The Scottish Government believes that the UN inspectors should be given the time and the full support of the international community to complete their investigations."
A team of UN weapons inspectors was back at the site of the attack on the outskirts of the capital Damascus today but they are able to look only at whether chemical weapons were used, not at who deployed them, and action could be taken before they have concluded their work.
Washington has said it will release further evidence of the regime's culpability - expected to involve intercepted signals intelligence.
UN secretary general Ban Ki-moon urged the members of the Security Council to act.
"The body entrusted with international peace and security cannot be missing in action," Mr Ban said.
"The Council must find the unity to act. It must use its authority for peace. The Syrian people deserve solutions, not silence."
Mr Cameron has warned that the world cannot stand idly by and must take firm action to show that the use of chemical weapons - in Syria or anywhere else - cannot be tolerated.
But he faces opposition to intervention from a number of his own backbenchers and polling shows the public is deeply reluctant for the UK to become embroiled in military action.
Former military chiefs have also issued stark warnings about the direction Mr Cameron is taking, cautioning that even a "surgical" missile strike could end up dragging the UK into deeper action.
And the Archbishop of Canterbury has urged MPs not to rush their decision, warning of the "unforeseeable ramifications".
Mr Cameron has insisted that any intervention in Syria would not be about the conflict itself, but preventing the use of chemical weapons by any regime and would be "proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons".
He held fresh talks with US president Barack Obama last night, after which No 10 said they were in no doubt that Assad's regime was responsible for the toxic assault.
Mr Ban said the inspectors would need a total of four days to finish their investigation as the team completed a second day of collecting samples and witness statements from the affected area.
The UN and Arab League special envoy to Syria Lakhdar Brahimi said the death toll from the "substance" used in the attack - widely thought to be the nerve agent sarin - could top 1,000
He said he was "very, very interested" to see additional evidence promised by Washington of what is described as the regime's "undeniable" involvement, which had not yet been shared with the UN.
And he cautioned that international law was "very clear" that military action required Security Council backing.
"What we have been told is that this evidence that the Americans, the British, the French say they have is going to be shared with us. It hasn't been until now," he said.
"And we will be very, very, very interested in hearing from them what this evidence they have is."
He went on: "I must say that I do know that President Obama and the American administration are not known to be trigger-happy.
"What they will decide, I don't know. But certainly international law is very clear: the Security Council has to be brought in."
His remarks came as Jordan declared that it would not allow its territory to be used to launch an armed response
Information minister Mohammad Momani said his country "will not be a launching pad for any military action against Syria" and urged the international community to "consolidate efforts" towards a diplomatic solution.