David Cameron has urged MPs from all parties to unite behind British air strikes against the Islamic State terror group in Syria.
Downing Street made clear that RAF bombing missions against IS - also known as Isil, Isis and Daesh - in Syria are likely to start within days if MPs back military action in a crunch vote on Wednesday evening.
Jeremy Corbyn's decision on Monday to allow his MPs a free vote looks certain to deliver Mr Cameron the clear majority for air strikes that he is seeking, with 50 or more Labour MPs expected to join Tories and Ulster unionists in backing action.
But Mr Corbyn insisted "more and more" Labour MPs were becoming sceptical about the proposals, warning: "We are not going to bomb our way to democracy."
Cabinet unanimously gave its formal approval to a motion which not only authorises air strikes in Syria, but also sets out plans to pursue a political solution to the Middle Eastern country's four-year civil war, commits Britain to humanitarian support and assistance with post-conflict reconstruction and rules out the deployment of UK ground combat troops.
Speaking after the meeting in Number 10, Mr Cameron said: "That motion talks about, yes, the necessity of taking military action against Isil in Syria as well as Iraq, but it is part of a broader strategy.
"It's about politics and diplomacy and humanitarian aid, all of which we need to bring to bear to bring peace to Syria but to make sure we protect our national interest of fighting against this appalling terrorist organisation."
The PM added: "I will be making the arguments and I hope as many Members of Parliament - across all parties - will support me as possible."
Mr Cameron's spokeswoman said that the mood at the weekly Cabinet meeting was "very serious", with the PM stressing "the complexities of the challenges we face, and the fact that this will require patience and persistence".
Ministers were briefing by Attorney General Jeremy Wright on the legal basis for action, which rests on the UN Charter right for the UK to defend itself and its allies.
Mr Cameron said that the prospect of putting members of the armed forces in harm's way "obviously preys very heavily on my mind", but said that he was following a "very deliberate and proper process" in securing political support.
His weekly session of Prime Minister's Questions on Wednesday has been cancelled to allow for 10 and a half hours debate in the Commons - which Downing Street pointed out was significantly longer than the seven and a half hours prior to the launch of strikes against IS in Iraq and the two and a half before the Falklands War.
The vote is expected around 10pm on Wednesday, but Labour continued to press for debate to be extended to two days, and Commons Speaker John Bercow said he was willing to "sit up all night" if MPs wanted.
Labour deputy leader Tom Watson said there was a "compelling case" for military action, but warned Mr Cameron in a letter that he was risking consensus by failing to provide sufficient "clarity" on his claim that 70,000 moderate rebels could provide ground forces to back up air strikes.
Mr Corbyn urged shadow cabinet colleagues planning to support the Government to "think again" and consider whether they want to commit British troops to "yet another war in the Middle East with no end-game in sight, no proper plan in sight, a mythical 75,000-strong apparently unknown Free Syrian Army operation which is also infiltrated by a lot of jihadist elements".
The Labour leader - who will open for Labour on Wednesday with a plea to avoid war, while shadow foreign secretary Hilary Benn wraps up the debate arguing the opposite - told BBC Radio 2's Jeremy Vine Show: "We are going to kill people in their homes by our bombs."
Mr Corbyn's decision to grant a free vote was branded "deplorable" by the Stop the War Coalition which he used to chair, who said it had "cleared the way" for the Commons vote. The group is planning a protest march in Westminster on Wednesday.
But Mr Benn insisted it was to the leader's "great credit" that he had allowed MPs to vote with their consciences, telling BBC Radio 4's Today: "People of principle can reach different decisions about how to deal with the threat."
Shadow energy minister and Corbyn supporter Clive Lewis said Labour MPs who chose to vote with the Government would have to face the consequences, telling Today: "If there are members of the PLP (Parliamentary Labour Party) that want to bomb in Syria and vote with the Tories, then on their heads be it."
Chancellor George Osborne told MPs the Syria campaign would cost in the "low tens of millions of pounds", compared to around £200 million a year already being spent on operations in Iraq and supporting the "moderate" rebels opposed to Bashar Assad in Syria.
Mr Osborne appeared to suggest he was expecting operations to begin within days, telling the Commons Treasury Committee: "I'm hoping that over the next couple of days' time, or whenever the action takes place, we will be flying jets alongside the French military ... These are our friends and allies"