Former UK Prime Minister Tony Blair has said that the Iraq war made MPs 'hesitant' to authorise military strikes in Syria.
Blair refused to concede that the failure to find weapons of mass destruction in Iraq, after the 2003 invasion, was a decisive factor in last week's House of Commons vote blocking UK intervention against Bashar al-Assad's regime.
However he admitted that armed interventions in conflicts which pit extremist Islamic factions against one another can be "very difficult."
Blair told the BBC: "It is an issue to do with the difficulty we encounter afterwards, and that is a really really important lesson.
"The truth is, the reason why Iraq makes us hesitant is because Iraq showed that when you intervene in the circumstances, where you have this radical Islamist issue, both on the Shia side and the Sunni side, you are going to face a very difficult, tough conflict."
Blair, who is a UN special envoy to the Middle East, claimed that the evidence that chemical weapons have been used in the Syrian civil war is undeniable, but MPs have not been convinced that Syria poses a threat to national security, a position he disputed.
He warned that Western inaction in Syria could lead to the country splitting into two, with one half becoming a haven for Islamist radicals and terrorists.
"You will have an Assad-dominated state, and that means in this instance an Iran-dominated state, probably around the borders of Lebanon and controlling most of the wealth of Syria," he said.
"And then you'll have a larger geographical hinterland to the east that will be controlled by various Sunni groups, most of whom are likely in these circumstances to be extreme, and you could have a breeding ground for extremism actually much worse and much more potent than Afghanistan."
There is a "fundamental battle about religion and politics within Islam", he continued, which "has vast consequences for our future security".
The UK and the US should support "the majority of people in the Muslim world [who] in fact want religion in a sensible place in politics, not trying to dominate politics", he said.
Blair's comments come as current Prime Minister David Cameron, who is attending the G20 summit in St Petersburg, pledged an extra £52 million of aid to Syria and other areas affected by the conflict, taking total UK aid to the region following the start of the civil war in 2011 to £400 million.
He said that the pledge constituted a "very large message of support" for Syrians affected by the conflict, and that the money would be used for training medics and providing supplies to treat victims of chemical weapons attacks.
The House of Commons vote has sidelined Cameron at the summit, where world leaders are divided on the best response to the Syria conflict.