Britain's parliament has backed its Prime Minisiter David Cameron and voted to approve airstrikes against Islamic State militants in Iraq.
The vote, which was passed by 524 votes to 43, will pave the way for the Royal Air Force to join U.S.-led military action with immediate effect.
Ahead of his vote, Cameron appealed to lawmakers, the majority of whom voted against military action in Syria last year, asking them: "Is there a threat to the British people? The answer is yes."
He also did not rule out the possibility of further military action beyond initial plans for airstrikes - and added that he would be prepared to order immediate intervention to stop a humanitarian catastrophe, without the backing of MPs.
The debate comes as airstrikes against the militants continue, with the U.S. targeting Syria and France – the first European country to join the coalition - striking ISIS targets in Iraq on Thursday, according to Reuters. A number of Arab allies are also involved.
As lawmakers returned to Westminster, Bethany Haines, the 17 year old daughter of David Haines, a U.K. aid worker murdered by Isis, made an emotional plea to do "what it takes" to stop the organization in a U.K. television interview. Meanwhile, the FBI claimed on Thursday to have identified the masked British jihadi who appeared in online videos of the killing of Haines and two American journalists.
MPs voted specifically on whether there is a "clear legal basis" for action in Iraq, but – importantly - not on "airstrikes in Syria".
However, Lord Malloch Brown, former UK government minister and UN Deputy Secretary-General, told CNBC earlier Friday that even though the motion was expected to pass, there would be "background anxiety" from MPs.
The arguments in favor
ISIS is actively targeting U.K. citizens in Syria and Iraq. It is also damaging the fragile state set up in Iraq by the U.K. and its allies over the last decade. And the group seems to be better organized and funded than its predecessor Al-Qaeda.
Also, Haider al-Abadi, the newly-appointed Iraqi Prime Minister, has directly asked for U.K. help - meaning that intervention is allowed under international law.
The military action will be restricted to firing on targets from the air, which will hopefully mean minimal risk to U.K. soldiers.
U.K. Foreign Secretary Philip Hammond told CNBC: "This will be an air operation. We are not expecting we will see anything like the casualties we have seen in previous conflicts where we have had ground forces involved."
The arguments against
There are concerns that the U.K., which only extracted itself from Iraq in 2011, is going to get stuck in another Middle Eastern conflict which could go on for years.
"There will be an underlying cross-party chorus (from UK MPs) of where's the strategy, where's the exit plan?" Malloch Brown added.
While the current emphasis is on no "boots on the ground", there is always the potential for this to change.
One of the more troubling aspects of recent fighting has been the presence of ISIS fighters who were born and raised in the U.K. There are concerns that further U.K. action in the Middle East could foster yet more discontent among young Muslims in the U.K.
- By CNBC's Catherine Boyle