The British Parliament is to be recalled for Friday for the Commons to approve air attacks on ISIL in Iraq. The resolution will not include approval for attacks in Syria. The proposal has the support of all three major parties.
Mr Cameron will open Friday's debate, due to start at 10:30 BST, and Mr Clegg will close it, with a vote expected at 17:30 BST.
Conservative MPs have had a three-line whip imposed on them, meaning they will be compelled to back air strikes. A Labour source said its MPs were expected to "support our position".
The cabinet is meeting to discuss the situation on Thursday.
Mr Miliband said: "We will be supporting the government on air strikes by Britain in Iraq.
"Isil is a murderous organisation. It's a murderous organisation against people of all nationalities and all religions. It is a threat directly to the stability of that region, and indeed to Britain because it can be a breeding ground for terrorism. I think is those circumstances we can't turn away from that threat."
During the parliamentary recess, ministers (but not ordinary MPs) have the power to request the Speaker that he recalls Parliament. If he decides that it is in the public interest that the Commons meets, he will decide the day and time and issue a notice to MPs.
The Commons will almost certainly approve the action. As well as the Conservative and Labour parties, the Liberal Democrats, who opposed the Iraq War in 2003, will vote for. The reasons were explained in an interview by Nick Clegg, the Deputy Prime Minister
The UK Attorney General to 2014, Dominic Greve, gave an interview on Newsnight (BBC Two, 10.30pm) in which he explained that action against ISIL in Syria could be legal as a consequence of the request for help by the Iraqi government, on the grounds that the attacks are coming from Syria. This is, however, not being pursued by the British.