Air strikes by British jets and drones against Islamic State (IS) in Syria look set to be given the green light by MPs as senior Labour sources conceded David Cameron is expected to win a crucial Commons vote.
The Prime Minister set out his case for military action against the "woman-raping, Muslim-murdering, medieval monsters" of IS, who he warned were "plotting to kill us and to radicalise our children right now".
The Commons faced a choice between backing RAF action against the group in its Syrian stronghold or sitting back and waiting for a terrorist attack on Britain's streets, said Mr Cameron. Delay would simply give IS time to grow stronger.
Critics of the plan disputed claims that 70,000 moderate fighters would be able to take on IS on the ground and Labour leader Jeremy Corbyn warned against an "ill-thought-out rush to war".
Mr Cameron's arguments for military action "simply do not stack up", said the Labour leader, who opposes military action but was forced to offer his MPs a free vote amid deep divisions within the shadow cabinet.
In a highly unusual move, shadow foreign secretary Hilary Benn will wrap up the debate for the Opposition with a speech in support of air strikes, hours after his leader condemned them.
Labour estimates of the number of MPs set to back military action despite Mr Corbyn's opposition and an occasionally hostile online campaign by activists ranged from 60 to more than 90, enough to deliver the Prime Minister a comfortable majority despite the likelihood of a small rebellion on his own benches.
Three former Labour ministers - Alan Johnson, Dame Margaret Beckett and Yvette Cooper - were among MPs making early speeches in favour of extending military action.
Former foreign secretary Dame Margaret said the UK had a duty to act following French appeals for support following the Paris atrocities.
"Consider how we would feel and what we would say if what took place in Paris had happened in London, if we had explicitly asked France for support, and France had refused," she said.
But former leader Ed Miliband, whose refusal to support Mr Cameron over strikes against Bashar Assad's chemical weapons in 2013 left the Prime Minister humiliated, said he would oppose military action.
At least 110 MPs from six different parties have already signed up to an amendment seeking to block air strikes, spearheaded by Tory John Baron and the Scottish National Party's Westminster leader Angus Robertson.
But Mr Cameron should be able to count on the votes of the DUP and Liberal Democrats.
Labour insisted that public opinion was turning against air strikes after a spike in support in the wake of last month's slaughter in Paris. A senior party source told reporters: "It's expected the Government will win the vote today, but it has lost the argument."
Mr Cameron said the Joint Intelligence Committee (JIC) and MI5 both assessed that the UK was among IS's "top tier" targets, while the Paris atrocities revealed the extent to which attacks in Europe were now being planned by "the head of the snake" in the terror group's HQ in Raqqa, Syria.
Spelling out the choice facing MPs, Mr Cameron said: "We face a fundamental threat to our security. Isil have brutally murdered British hostages, they have inspired the worst terrorist attack against British people since 7/7 on the beaches of Tunisia, and they have plotted atrocity after atrocity on the streets here at home.
"Since November last year our security services have foiled no less than seven different plots against our people. So this threat is very real and the question is this: Do we work with our allies to degrade and destroy this threat and do we go after these terrorists in their heartlands from where they are plotting to kill British people, or do we sit back and wait for them to attack us?"
Britain must not allow the "past mistakes" of the 2003 Iraq War to become "an excuse for indifference or inaction", he said.
"Let's be clear - inaction does not amount to a strategy for our security or for the Syrian people, but inaction is a choice," said Mr Cameron. "I believe it's the wrong choice."
The PM repeatedly refused to apologise after urging Tory MPs not to side with "terrorist sympathisers" in opposing air strikes. Mr Corbyn said the remarks "demean the office of prime minister".
Critics of air strikes - including the 54-strong SNP - were emboldened by criticism of Mr Cameron's case from the influential Commons Foreign Affairs Committee and controversy over his claims of 70,000 moderate Syrian forces on the ground.
The Conservative chairman of the Commons Defence Committee, Julian Lewis, said that in place of the "dodgy dossiers" used by Tony Blair's administration to justify war in Iraq in 2003, "we now have bogus battalions of moderate fighters".
Mr Cameron acknowledged that the fighters in a disparate array of rebel groups were not "ideal partners" in the fight against IS.
But he defended the figure as the estimate of the JIC - the UK's senior intelligence body - and said the majority were members of the Free Syrian Army, while there were a further 20,000 Kurdish fighters with whom Britain could also work.
And he said MPs should not underestimate the impact which RAF strikes could have, saying that they represented between a quarter and a third of the international coalition's typical precision bombing force and a quarter of the unmanned drone capability in the region. In more than a year of action in Iraq, there were no reports of RAF strikes causing civilian casualties, and the skill of British pilots would make it less likely that innocent people would die in Syria, he said.
Mr Corbyn disputed Mr Cameron's claim about ground troops, saying it was "quite clear there are no such forces" and only extremists would take advantage of the strikes against IS.
He said the UK's involvement was "unlikely to make a huge difference" militarily and would lead to "mission creep" which could end with British troops on the ground in Syria - something which is explicitly ruled out in the Government's motion.
The Labour leader - who repeatedly ducked challenges to say whether he would halt air strikes in Iraq - urged MPs to "step back and vote against yet another ill-fated twist in the never-ending war on terror".
Opposing intervention was not pacifism but "hard-headed common sense", he said.