As supporters of the Stop the War Coalition gather outside the gates of No 10 today, the Prime Minister will be inside trying to draw up a parliamentary motion that gives him the best chance of majority support.
Without it the UK's involvement in any action against Syria would be politically explosive, if not impossible.
Yesterday, the PM stressed how any response had to be "proportionate, legal and would have to specifically be about deterring the use of chemical weapons".
Nick Clegg, his deputy, said: "Any steps being taken will have to be legal. This Coalition Government, of course, is not going to act outside the remit of international law but, let's remember, that the use of chemical weapons is a flagrant abuse of international law."
However, it has been argued, most notably by Russia, that for military intervention to be lawful, it would have to be sanctioned by the UN Security Council; which it will not be because Moscow would veto any resolution.
Richard Ottaway, the Conservative chairman of the Commons Foreign Affairs Committee, acknowledged that acting without a UN resolution would lack "legal authority" but he suggested action would be "legitimate" due to the of scale of atrocities.
However, Geoffrey Robertson, QC, the leading international lawyer, claimed it would be wrong to argue that military intervention would be unlawful without the backing of the Security Council.
The former UN appeal judge explained that it would be necessary first to establish evidence of the Assad regime's guilt for the chemical weapons attack and noted: "There is a right for regional groups like Nato and the Arab League to use force to stop crimes against humanity such as a state mass-murdering its citizens by poison gas."
Richard Haas, President of the US Council on Foreign Relations think-tank, also argued that a Western attack on Syria would not necessarily need Security Council approval to make it legal.
He said: "The UN Security Council is not the sole or unique custodian about what is legal and what is legitimate, and, as many have pointed out, it was bypassed at the time of Kosovo."
He explained authority could come from a "coalition of the willing", of individual countries that supported retaliation against Syria to show that the use of weapons of mass destruction would not be tolerated.
Article 51 of the UN Charter refers to "the inherent right of individual or collective self-defence if an armed attack occurs against a member of the United Nations".
Meantime, it could be argued that military action would also be justifiable under the UN's "responsibility to protect" protocol, because Syria had failed to protect its own population from atrocities.
The legal arguments will be at the forefront of tomorrow's Commons debate.
Tory MP Robert Halfon said: "I very much hope politicians of all parties will vote to stop mass genocide and the use of chemical weapons."
But his Conservative colleague Andrew Bridgen stressed how MPs would need assurances "on the grounds for action, that there is compelling evidence it is the Assad regime that launched the chemical attacks ; we will need the aims of any action and limits and scope of action, and information on who else will be involved".
l Tomorrow's recall of Parliament will be the fourth time Mr Cameron has interrupted MPs' holidays.
He's did it to allow tributes to former Prime Minister Margaret Thatcher earlier this year and twice in the summer recess of 2011; first for a statement about phone hacking and the police in July and later following the London riots.