Britain's parliament looks set to vote in favour of joining the bombing campaign against Islamic State (ISIS) in Syria on Wednesday, paving the way for sorties by its planes to start within days.
Prime Minister David Cameron, who stepped up pressure for air strikes after last month's Paris attacks, will lead the House of Commons into more than 10 hours of debate on joining the US-led international military action.
Ministers are confident that lawmakers will then vote "Yes", meaning the first Royal Air Force (RAF) planes could be bombing targets in Syria by the end of the week.
Cameron insists military action is needed to prevent attacks like the ones that killed 130 people in Paris last month, while insisting it will be accompanied by a diplomatic push to resolve the crisis in Syria.
"I will be making the arguments and I hope as many members of parliament across all parties will support me as possible," he said on the eve of the vote.
But many experts, lawmakers and members of the public remain sceptical, and several thousand anti-war protesters marched in central London on Saturday and Tuesday.
Military experts question how much difference Britain will make to the coalition against ISIS Militants in Syria, adding the move may be more about Britain wanting to be seen to stand shoulder-to-shoulder with allies like France and the United States.
"It will not make a big operational difference," Professor Malcolm Chalmers of military think-tank the Royal United Services Institute (RUSI) told AFP.
"It is important symbolically, useful operationally, but not transformative."
Cameron "emotionally feels very strongly that he should support France in its time of need," added Ben Barry of the International Institute for Strategic Studies think-tank.
Britain already has eight Tornado fighter jets plus an unknown number of drones involved in strikes on ISIS targets in Iraq, an operation it joined last year.
However, it currently only conducts surveillance and intelligence missions over Syria.
In a rare move, weekly Prime Minister's Questions have been cancelled and the parliamentary diary cleared for an all-day debate on the Syria strikes, with a vote expected around 2200 GMT or even later.
The motion up for debate stresses that Britain will not deploy ground combat troops while noting that allies including France and the United States had requested British assistance.
Cameron announced the vote on Monday after the main opposition Labour party let its MPs have a free vote rather than trying to force them to oppose air strikes in line with the views of its left-wing leader Jeremy Corbyn.
Labour's decision means dozens of its MPs who want military action will now likely vote with Cameron and the government, meaning he should get the "clear majority" he said was essential for action.
However Labour remains deeply divided, prompting Corbyn to take to the airwaves Tuesday in a last-ditch appeal to his own MPs to vote against air strikes.
"We are going to kill people in their homes by our bombs," he told BBC radio.
"Think of the complications and the implications of what we are doing and please cast your vote against supporting this government's military endeavours in Syria."
In a blow to Cameron on the eve of the vote, parliament's foreign affairs committee said he had "not adequately addressed" its concerns on Syria air strikes.
The committee has cast doubt on the legality of the move, its effectiveness in the absence of reliable allies on the ground and its usefulness in the context of finding a diplomatic solution to the crisis.
While Cameron is expected to win, he will likely face fresh questions from MPs about his claim that there are 70,000 moderate opposition fighters in Syria ready to help secure territory following air strikes.
Scarred by the memory of unpopular wars in Afghanistan and Iraq, London has played a smaller role in recent foreign military actions, leading to concerns that Britain's clout is diminishing on the world stage.
Parliament rejected Cameron's plan for action against the Assad regime in Syria in 2013.
"A willingness to deploy will allay the concern the UK is not a reliable partner," RUSI's Chalmers said.
A YouGov opinion poll last week found that 59 percent approved of Britain joining air strikes in Syria, compared to 20 percent who disapproved, but a survey published on Wednesday showed only 48 percent approved with 31 percent against.