Tony Blair has said the aftermath of the invasion of Iraq has made the UK "hesitant" to intervene in Syria.
It was not that MPs did not trust the government's assessment of the threat posed by Syria, as chemical weapon use had been proved, he told BBC Radio 4.
The former prime minister added he was disappointed the UK would not be taking part in military action.
And he "disagreed" with Labour leader Ed Miliband, who helped defeat the government in a Commons vote last week.
A Labour source rejected Mr Blair's analysis, saying the lessons the Labour leader had learned from Iraq was the importance of avoiding an "ill-judged and reckless rush to war".
But former cabinet minister Ben Bradshaw has said many Labour MPs felt "uneasy" that Britain's options had been limited after last week's vote.
Writing in the Guardian, the former culture secretary blamed David Cameron for "petulantly" ruling out direct intervention immediately after losing the vote.
But he also expressed alarm that Mr Miliband was following the prime minister's lead rather than taking a "sensible and measured approach".
"We were not voting to support Britain taking part in immediate military action, but nor were we voting to rule it out completely. Neither were most Conservatives or Liberal Democrats," said Mr Bradshaw.
In a message on Twitter, Labour former deputy prime minister Lord Prescott said: "I've always respected Tony Blair but he's wrong on Syria and Ed is right. Tony seems to have become a champion for regime change."
Explaining his opposition to Mr Miliband's stance, Mr Blair told Radio 4's What Syria Means For Britain. "This is something where I just have to disagree with the leadership of the party.
"I know it's a difficult position for political leaders to be put in when they've got to take decisions like this, but my position on these issues is pretty clear over a long period of time."
In last Thursday's debate, Mr Cameron told MPs: "The well of public opinion has been well and truly poisoned by the Iraq episode."
But, echoing Mr Blair, he said Syria was not like Iraq, because "the fact that the Syrian government have, and have used, chemical weapons is beyond doubt".
Explaining his reasons for backing military action, Mr Blair said not acting in Syria would be "dangerous" as it would "send a signal" that chemical weapons could be used without a "robust response".
Without foreign intervention, he warned "you will have a [President Bashar al-]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 [Muslim] 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."
He added: "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 [Muslim] side and the Sunni side, you are going to face a very difficult, tough conflict."
There was a "fundamental battle about religion and politics within Islam", he said, 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.
Mr Blair, now Middle East envoy for the Quartet of the United Nations, the United States, the European Union and Russia, said the Commons vote would have an impact on US-UK relations.
"What the long-term implications of that are depends on future actions," he said.
Former Lib Dem leader Sir Menzies Campbell, who opposed the Iraq war in 2003, questioned Mr Blair's analysis.
"Mr Blair's contention that public disquiet in the UK in relation to military action against Iraq can be put down to the aftermath of military action is undermined by three facts.
"First, the military action was justified on the basis that Saddam Hussein possessed weapons of mass destruction which were a danger to the UK, which was just plain wrong.
"Second, that the war against Saddam Hussein was illegal and contrary to the UN charter.
"Third, that the intelligence presented to Parliament and public was couched in such a way as to fit our policy decision already taken wholly contrary to principle.
"The aftermath certainly didn't help public perception but it was not the only compelling reason for public disillusionment."
It comes as David Cameron announced a further £52m in UK aid for victims of the civil war in Syria.
It will bring the UK's total spending on aid for Syria and neighbouring states to £400m.
What Syria means for Britain will be broadcast on Monday evening at 20:00 on BBC Radio 4.