Britain could be at war by the weekend, with Parliament expected to be recalled on Friday to approve UK air strikes against the Islamic State.
The UK now looks more likely than ever to join the US strikes, though it is likely that UK help will be focussed on Iraq rather than Syrian targets which were hit yesterday by the US and five other Arab partner nations.
The Guardian reported that RAF Tornado jets based in Cyprus are being prepared to bomb IS targets in Iraq, to be ready the moment Cameron gives them the order. Six Tornado GR4 aircraft have been flying reconnaissance missions over northern Iraq and an RAF Rivet Joint surveillance plane equipped with listening devices has also been flying missions from al-Udeid air base in Qatar to intercept intelligence.
Ed Miliband indicated he would not support British military action against Isis in Syria unless there was a United Nations security council resolution authorising it. "We support the overnight action against Isil [Isis]. What needs to happen now is that UN needs to play its part - a UN Security Council resolution to win the international support to counter that threat of Isil," he said in his leader's speech to Labour conference on Tuesday.
But he appeared to soften his stance in broadcast interviews on Wednesday, saying his party was "open to the possibility" of UK forces joining air strikes in Iraq but would not commit in advance of seeing detailed proposals.
He said he would "look very seriously" at any proposal to meet a request from Iraq for British involvement, but action in Syria was a "more complex" question. "Isil is a threat that can't be turned away from," he told BBC Breakfast. "We've said that we need an alliance which is not just a military alliance but a political and diplomatic alliance. How will I judge any proposal? Whether Britain can have an effect, whether we can succeed and whether it is legitimate and lawful. But I am open to the possibility. This is a very grave decision as someone who wants to be the prime minister. Before I commit British combat troops I want to look at what the proposition is and the nature of that proposition."
He said he had not spoken to Cameron about the issue or the potential recall of Parliament. Asked about action in Syria, he said he understood why the US had intervened there but renewed his call for a United Nations Security Council resolution to give the action "proper legitimacy" in the absence of a request from the Assad regime.
Ahead of David Cameron's address to the United Nations in New York later on Thursday, the Prime Minister is likely to receive a formal Iraqi request for UK involvement at a meeting with Haider Abadi, Iraq's new premier.
Later, he will become the first British Prime Minister since the 1979 Islamic Revolution to hold face-to-face talks with an Iranian president, and will seek to enlist Iraqi president Hassan Rouhani in the fight against IS.
Speaking to US TV channel NBC, the PM said IS posed a direct threat to Britain and the West. "So this is a fight you cannot opt out of. These people want to kill us. They've got us in their sights and we have to put together this coalition ... to make sure that we ultimately destroy this evil organisation."
There was "no doubt in my mind" that IS was to blame for the murder of four people at a Jewish Museum in Brussels in May and was planning further outrages in the West, he said. "There are other plots they have been attempting, including in my own country, in order to kill and maim innocent people," said the PM. "And the same applies to the United States of America.
Iraq has not yet formally asked Britain to join the US and France in air strikes on IS forces which have seized control of large swathes of northern Iraq, though the UK responded swiftly to Baghdad's plea for heavy machine guns and ammunition earlier this month and Downing Street aides indicated that Cameron was ready to consider further requests from Mr Abadi when they meet for the first time at the UN building.
A request from the Baghdad government would provide legal underpinning for UK strikes in Iraq - something which is not so clear cut in the case of attacks on IS strongholds in Syria, where the UK has made clear it will not co-operate with the regime of president Bashar Assad.
Any offer of military help to Iraq could lead to a recall of Parliament on Friday to seek MPs' approval, with Cameron hopeful of avoiding the defeat he suffered last year over plans to target the Assad regime.
Cameron is also set to back a sweeping new UN resolution tabled by the US, which would impose global travel bans on foreign fighters planning to join conflicts overseas, as hundreds of Britons are believed to have done in Syria and Iraq.
The move, being personally pushed by Obama at a Security Council meeting attended by Cameron, would require UN member states to update national laws to stem the flow of foreign fighters, including by withholding passports and freezing assets.
The Pentagon said that a wave of bomb and missile attacks on targets in Syria and Iraq on Monday night was prompted in part by fears that a shadowy al Qaida-linked organisation known as the Khorasan Group was preparing to mount "imminent" terror attacks on Western targets.
The first US raids on targets in Syria involved Tomahawk cruise missiles fired from US warships at IS compounds, workshops and training grounds around the city of Aleppo, ahead of warplane and drone strikes on positions elsewhere in the country.
Among the areas reportedly hit was the IS stronghold of Raqqa where it is thought British aid worker Alan Henning is being held.
Henning's brother-in-law Colin Livesey said he was "scared" about the possible impact of the air strikes on the former taxi driver's safety.
"If they're going to do air strikes on them, they'll just run away," Livesey told ITV News. "They'll take him with them and no-one will know where he is again. "He thought he was doing the right thing by going up there and helping other people in another country, to give them aid, because that's the sort of guy he is. He just went there pure and simple to help kids out. That's all he did it for. He doesn't deserve what's happening to him now."
Henning's wife Barbara urged Islamic State to release him after she received an audio message from her husband pleading for his life.
Barbara Henning called for the militant group to "open their hears and minds" and said she had been told that a Sharia court had found her husband innocent of being a spy. "I implore Islamic State to abide by the decisions of their own justice system. Please release Alan," she said in a statement released through the Foreign Office.