Alastair Campbell has warned it would be "hugely irresponsible and incredibly dangerous" if the world does not intervene in the conflict in Syria.
Campbell, director of communications for Tony Blair when he was prime minister, said Syria was in the throes of a "humanitarian catastrophe" well before the chemical weapons crisis.
Speaking on ITV's Daybreak, he said: "And I think even before the use of his chemical weapons, which even the Arab League now seem to have accepted the evidence on that, even before that, this was a humanitarian catastrophe that frankly the world has turned away from for far too long.
"But to have somebody using chemical weapons against his own people, in the way that Assad has done, on top of all else that he's done, I think for the rest of the world to stand by and just say 'Oh well, carry on, and do it again and we won't do anything and we won't get involved,we'll just stand to one side' - I think that would be hugely irresponsible and incredibly dangerous."
Campbell's comments came the day after his former boss, Blair, accused the regime of Bashar al-Assad of launching an attack on civilians on a scale "not seen since the dark days of Saddam" and said the West needed to intervene.
He said: "Western policy is at a crossroads: commentary or action; shaping events or reacting to them. After the long and painful campaigns in Iraq and Afghanistan, I understand every impulse to stay clear of the turmoil, to watch but not to intervene, to ratchet up language but not to engage in the hard, even harsh business of changing reality on the ground. But we have collectively to understand the consequences of wringing our hands instead of putting them to work."
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Campbell, who was intimately involved in the decision to take part in the 2003 invasion of Iraq, said people should not envy the position David Cameron, and other world leaders, find themselves in. He said: "Put it this way, I think it is very, very hard for the world to stand aside, given what we know has happened."
He emphasised the responsibility the rest of the world has in playing its part, and said if the concept of an international community holds any meaning, then it must not ignore what is happening in Syria.
And he downplayed the role of the UN in terms of any actions needing to be sanctioned, saying that the UN is more a political body than a judgment body.
He said: "And when you have a situation where the Russians, in particular, are standing out and saying they're just not going to support anything, then I think it is incumbent upon the Americans, the British, the French and others to say we are going to respond to this, we are going to respond to what has become already a humanitarian catastrophe."
Blair and Campbell's comments stand in contrast to current senior Labour front bencher Diane Abbott, who could quit Ed Miliband's front bench if the party is asked to vote in favour of military intervention.
"I voted against the Iraq War. At the moment, I can't see anything that would make me vote for intervention in Syria," she told The Guardian yesterday.
Yesterday Miliband said the Labour Party may back possible military strikes against the Syrian regime, after David Cameron announced parliament would be recalled to allow MPs to vote on what action to take.
However today Labour said it has made clear to Cameron that its support depends on assurances that fresh efforts will be made to secure United Nations backing.
A senior source told the Press Association: "As part that legal justification, Labour is seeking the direct involvement of the UN through evidence from the weapons inspectors and consideration by the security council."