Tony Blair has attacked Labour leader Ed Miliband's position on military action against Syria.
The former prime minister said that he disagreed with Mr Miliband's handling of the issue, warning that the troubled country could become a breeding ground for extremists unless Bashar al-Assad's regime was confronted.
Speaking to the BBC, Mr Blair said he was "disappointed" that the House of Commons had rejected the Government opening the way for military intervention.
“This is something where I just have to disagree with the leadership of the party,” he said. “I know it's a difficult position for political leaders to be put in when they have got to take decisions like this.”
Mr Blair, who left Downing Street in 2007 after ten years as premier, also argued that “protracted” difficulties following the invasion of Iraq had made the UK hesitant to intervene in Syria.
He suggested that it was the aftermath of the invasion, rather than the failure to find weapons of mass destruction in Iraq, that influenced Parliament's decision to block military action in Syria.
He also spoke of a “fundamental battle” within Islam that could have widespread consequences for the UK's future security.
While he conceded that interventions can be “very difficult”, he was strident in his view that Syria, left unchecked, could become a potent source of extremists.
Mr Blair, currently Middle East envoy for the so-called Quartet of the UN, the United States, the EU and Russia, warned that without intervention there would be 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.
“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.”
The former Labour leader contended that there was no question that chemical weapons had been used in Syria, and so he believed that British MPs had not voted against interventions because they did not trust the government's assessments of the threat posed by Syria.
“It is an issue to do with the difficulty we encounter afterwards, and that is a really really important lesson,” he said.
“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.”
There is a “fundamental battle about religion and politics within Islam”, he continued, which “has vast consequences for our future security”.
Responding to Mr Blair's intervention, a Labour source told The Independent, “We have learnt the lessons of the Iraq War. That is why Ed was determined to stop David Cameron's ill-judged and reckless rush to war.”
The comments followed Prime Minister David Cameron's announcement of a further £52 million in British aid for victims of Syria's civil war.