David Cameron is preparing to hold a parliamentary vote on extending air strikes to Syria by Christmas, after saying there is a "compelling case" in the wake of the Paris terror attacks.
Britain needed to target the "head of the snake" with strikes on Raqqa, the Islamic State of Iraq and the Levant's headquarters in Syria, to deal with the "direct and growing threat to our country", the Prime Minister said.
He told the House of Commons that the case for intervention had "grown stronger" after the killing of 129 people in the French capital. "After the horror must come our resolve and determination to rid the world of this evil," Mr Cameron said. "It is in Syria, in Raqqa, that Isil has its headquarters and it is from Raqqa that some of the main threats against this country are planned and orchestrated. Raqqa, if you like, is the head of the snake."
Mr Cameron added: "In this situation, we do not protect the British people by sitting back and wishing things were different. We have to act to keep our people safe."
It is understood that he is prepared to hold a vote on Syria next month if it can win enough support from MPs. He has been emboldened by disarray within the Labour Party over Jeremy Corbyn's stance on terrorism and by public outrage over the Paris attacks.
Within days, the Prime Minister will publish a dossier setting out the case for air strikes in Syria, in response to concerns raised by the Commons foreign affairs committee.
The Ministry of Defence will then hold briefings with MPs from all parties as the Government seeks to build support for further intervention.
Government sources said that they expected around 20 Conservative MPs to rebel in a Commons vote on Syria but were optimistic that they could win support from up to 30 Labour MPs. Francois Holland, the French president, has said he wants a United Nations Security Council resolution to "destroy" Isil.
The US president, Barack Obama, has called on other nations to "step up" efforts against the extremist group.
Mr Cameron is likely to want a parliamentary mandate for Britain to help pass such a motion in the UN. His Commons address came as:
Russia mounted strikes on Isil targets in Raqqa after President Vladimir Putin said he wanted "vengeance" for the downing of a Russian airliner over Egypt's Sinai peninsula.
French authorities said they were searching for a second terrorist on the run after the Paris attacks.
Mr Corbyn, the Labour leader, was forced to backtrack after suggesting he was "not happy" with a "shoot-to-kill" policy in the event of a terror attack in the UK.
The Muslim Council of Britain took out a newspaper advert condemning the Paris attacks "unreservedly".
A football match between Germany and the Netherlands was called off an hour and a half before kick-off after police found "concrete evidence" of a bomb threat.
The Ministry of Defence yesterday confirmed that a British drone helped French jets carry out deadly air strikes against Isil in Syria at the weekend.
Ministers also disclosed that on Monday RAF Tornados used laser-guided bombs to find and kill 30 Isil fighters preparing to attack Kurdish forces in Iraq. But Britain has been unable to extend military action into Syria since Mr Cameron was defeated in a 2013 Commons vote, when Tory rebels joined Labour MPs to oppose air strikes against the Assad regime.
Mr Hollande will meet Mr Obama in Washington next week, then travel to Moscow to see Mr Putin as he seeks to build the international coalition against Isil.
Sylvie Bermann, the French ambassador to the UK, has said it would be "appreciated" if the RAF carried out raids in Syria alongside the French. Mr Cameron said: "The case for doing so has only grown stronger after the Paris attacks. We cannot expect, we should not expect, others to carry the burdens and risks of protecting our country."
France also became the first country to invoke an article of the Lisbon Treaty calling for other EU nations to offer "aid and assistance by all means in their power".
Jean-Yves Le Drian, the French defence minister, said they could help "either by taking part in France's operations in Syria or Iraq or by easing the load or providing support for France in other operations".
Last month the Commons foreign affairs committee set out seven conditions for British air strikes in Syria, including winning support from Iran and Saudi Arabia.
But Crispin Blunt, the Conservative committee chairman, adopted a more conciliatory tone yesterday.
He said: "I think we're beginning to work towards the kind of international plan that will mean we've got a military task that we can achieve."
Mr Corbyn said that the Prime Minister should not "feed a cycle of violence and hatred" with his response to the terror attacks.
Hilary Benn, the shadow foreign secretary, made clear he did not agree with Mr Corbyn, while moderate Labour MPs used questions in the Commons to criticise their leader.
The Royal United Services Institute, a military think tank, says today that the case for Britain joining strikes is "in important respects, stronger" after the Paris attacks, with pressure building to "resolve this issue one way or another".
It warns that any military campaign "may have to be sustained over a period of several years".