UK MPs have overwhelmingly backed airstrikes against Islamic State (IS) fighters in Iraq - meaning RAF bombing raids could take place in hours.
The British parliament gave approval by 524 votes to 43 for Britain to join the US-led coalition in the Middle East after the UK Prime Minister David Cameron said IS forces are "psychopathic terrorists trying to kill us".
Labour MP Rushanara Ali immediately resigned from the party's front bench after the result was announced.
Labour leader Ed Miliband told her afterwards "I know that you have thought long and hard about this. I respect and accept your resignation."
Mr Cameron told the Commons debate that Islamic militants "have already murdered one British hostage" and are "threatening the lives of two more".
He described IS, which has invaded large areas of Syria and Iraq, as "a terrorist organisation unlike those we have dealt with before".
He said: "The brutality is staggering - beheadings, crucifixions, the gouging out of eyes, the use of rape as a weapon, the slaughter of children. All of these things belong to the dark ages."
"This is not a threat on the far side of the world. Left unchecked we will face a terrorist caliphate on the shores of the Mediterranean and bordering a NATO member with a declared and a proven determination to attack our country and our people."
The PM said the shadow of the 2003 decision to join the US-led invasion of Iraq "hangs heavy" over the vote, but told MPs: "We must not use past mistakes as an excuse for indifference or inaction."
Britain has six Tornado GR4 fighter bombers in Cyprus ready to strike northern Iraq, a figure which Cabinet minister Kenneth Clarke said would make the UK's military contribution "almost symbolic".
The planes have been in Cyprus for the past six weeks carrying out surveillance missions in the Middle East.
During the debate, Mr Miliband said he understood the deep unease about taking military action, but said the UK could not stand by in the face of the threat from IS, also known as ISIL.
"ISIL is not simply a murderous organisation; it has ambitions for a state of its own - a caliphate across the Middle East, run according to their horrific norms and values," he said.
But outspoken Respect MP George Galloway said bombing would not work, and stressed the need to strengthen ground forces in the region.
He said: "ISIL is a death cult, it's a gang of terrorist murderers. It's not an army and it's certainly not an army that's going to be destroyed by aerial bombardment."
On Thursday, the UK Cabinet unanimously backed military action against IS, which could last up to three years.
Mr Cameron was desperate to avoid the embarrassment of the Commons defeat on Syria airstrikes last year - and tabled a cautiously-worded motion intended to win support from all parties for action in Iraq.
He went into the debate with an opinion poll suggesting voters strongly back Iraq airstrikes, but would also support attacks against Syria.
Meanwhile, Denmark announced it was sending seven F-16 planes to join the US-led coalition against IS.
