A British military attack on Syria will have to be a "judgment call" as there is no "single smoking piece of intelligence" that the regime used chemical weapons, David Cameron said at the beginning of the emergency Commons debate on Syria.

Arguing in favour of military intervention, the prime minister said he believed that forces loyal to Bashar al-Assad did use poisonous gas against the Syrian people "right in front of our eyes".

Cameron told MPs there were at least 95 "horrific videos" of people dying in a gas attack in a suburb of Damascus two weeks ago, and added there was further evidence in the form of witness and social media reports.

However, the prime minister did concede that there could be no 100% certainty about the intelligence on which Britain would have to make a decision on whether to intervene in Syria.

Cameron argued it was in Britain's national interest to maintain the "international taboo" against the use of chemical weapons on the battlefield. He said the consequences of inaction would be greater than action, as it would give Assad a green light to commit more abuses. "We must not let the spectre of previous mistakes paralyse us," he added.

He acknowledged people were sceptical about getting into "another war in the Middle East" after the actions of the Blair government in the runup to the 2003 Iraq war "well and truly poisoned the well of public opinion" about intervention on the basis of secret intelligence. But he insisted the situation was fundamentally different to Iraq.

"I am of course deeply mindful of the lessons of previous conflicts and in particular the deep concerns in the country caused by what went wrong with the Iraq conflict in 2003," he told MPs during a packed House of Commons debate on the issue.

"But this is not like Iraq. What we are seeing in Syria is fundamentally different," he said. "We are not invading a country. We are not searching for chemical or biological weapons. The case for ultimately, and I say ultimately because there would have to be another vote in this house, the case for ultimately supporting action is not based on a specific piece or pieces of intelligence."

He said the government's motion was purely based on the argument that the world must not stand by while chemical weapons are used against civilians. "The motion also makes clear that, even if all these steps are taken, anything we do would have to be legal, proportionate and specifically focused on deterring further use of chemical weapons," he said.

The prime minister promised he would wait until UN weapons inspectors had delivered their verdict before joining the US in any intervention. The inspectors are due to leave the country on Saturday and, it is understood, could deliver an interim report to the United Nations that day.

He also said it would be "unthinkable" to proceed if the UN security council was overwhelmingly against an attack – although this was not thought to refer to Russian and Chinese objections.

No military strike will take place without another vote by MPs in the House of Commons, Cameron added. "I wanted the recall of this house to debate these absolutely vital national and international issues," he said.

"It is this house that will decide what steps we next take. If you agree to the motion I have set down no action can be taken until we have heard from the UN weapons inspectors, until there has been further action at the United Nations and another vote in this house. "Those are the conditions we the British government, the British parliament, are setting and it is absolutely right we do so."

Ed Miliband said Cameron had not yet made a convincing enough case for a military strike on Syria. He said evidence published on Thursday by the joint intelligence committee was "important" but more was needed for it to be conclusive.

The Labour leader made it clear he was not opposed to action in principle but wanted more time and better arguments before his MPs would give the coalition their backing. The prime minister must make "a better case than he did today", he added.