Speaking to The Huffington Post UK on Wednesday on the eve of a high-stakes Commons vote that would authorise retaliation against the Assad regime for the alleged use of chemical weapons, Cheryl Gillan said she would did not want to repeat the mistake of Iraq and was "suspicious" of the case for war.
Gillan served as Cameron's Welsh secretary until September 2012 when she lost her job in government at the reshuffle.
"I voted for the Iraq war after I listened to Tony Blair, he turned out to be leading us all up the garden path. I don't want to do that again," she said.
"I very much worry we haven't got the provenance on these chemical attacks. I am just horrified as anyone. I just feel the helplessness of anyone being attacked by chemical weapons. I don't want to diminish that, but we have to make sure who launched that attack and why."
Gillan added: "What will be the unintended consequences? What will be the diplomatic consequences? What will be the fall out as far as other countries are concerned? I'm worried we are embarking on a line which could lead to absolute disaster."
On Tuesday Cameron announced he was recalling parliament on Thursday to give MPs a debate and vote on any action as he had previously promised to do. The fact parliament was due to return on Monday anyway has increased suspicions the UK and US intend to launch strikes before the end of this week.
"There are too many unknown quantities," Gillan said. "I am worried because parliament has been recalled two days before it was due to come back anyway. This indicates to me the Americans have already committed and we are getting in their as fast as possible to support our allies.
"If you were thinking about it from the prime minister's perspective; if he's given some sort of commitment to the Americans … he had also promised that parliament would have a vote on it, he would want to sticks to his word.
Asked if her view was shared by other Conservative MPs, the veteran Chesham and Amersham MP, who served as a shadow Foreign Office minister when William Hague was Tory leader, said:"I don't think mine will be very different to many of my colleagues."
A significant number of backbench Conservative MPs have frequently expressed deep concern about any move to intervene militarily in Syria - and the prime minister is unlikely to be able to rely on all his MPs to support him in tomorrow's vote.
Earlier this year 81 Tories signed a letter to Downing Street demanding they be given the right to veto any military action.
Gillan said failing to have a proper Commons debate and vote would have looked "very bad particularly in the light of what happened in Iraq".
She also pointed out that the Chilcot Inquiry, which is examining the run-up to the 2003 invasion, has still not reported back.
"We don't even have the final position into Iraq," she said. "Call me cynical but when Tony Blair came out in support [of intervention in Syria] I started to have another look at it."
Gillan said she had been "utterly convinced" by Blair's arguments in favour of the Iraq War at the time and put aside party politics believing a "British prime minister would never ever let us have the opportunity to launch military action without it being based on sure foundations."
She added: "I'm older and wiser now, I'm a lot more suspicious."
On Tuesday a YouGov poll for The Sun showed 50% of Britons oppose attacking Assad's regime with long range missiles from ships, while just a quarter are in favour of it.
Gillan said the legacy of the wars in Iraq and Afghanistan were partially responsible for voters' caution.
"I think people in the country have instincts. And I think the instincts of the British people at the moment are really cautious and not to intervene," she said. "I think Iraq and Afghanistan have blunted the appetite for the UK to be the policeman of the world. And I think peoples instinct is still super cautious towards any intervention directly by the UK."
Cameron announced this morning that Britain will put a resolution to the United Nations Security Council later on Wednesday "authorising necessary measures to protect civilians" in Syria. The prime minister announced the move on his Twitter feed on Wednesday morning. "We've always said we want the UN Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that," he said. The resolution is highly unlikely to pass as Russia and China will block it.