Since David Cameron made his case for extending UK bombing to Syria in the House of Commons last week, that case has been coming apart at the seams. No wonder he is trying to hurry the debate through parliament this week.

He knows that opposition to his ill-thought-out rush to war is growing. On planning, strategy, ground troops, diplomacy, the terrorist threat, refugees and civilian casualties, it’s become increasingly clear the prime minister’s proposal simply doesn’t stack up.

That’s why the respected House of Commons foreign affairs select committee – whose critical report on his bombing plans was the focus of the prime minister’s statement – tonight made clear he had not adequately addressed their concerns.

After the despicable and horrific attacks in Paris last month, the issue of whether what Cameron proposes strengthens – or undermines – our own security is crucial. There is no doubt that the so-called Islamic State (Isil) group has imposed a reign of terror on millions in Iraq, Syria and Libya. And there is no doubt that it poses a threat to our own people. The question is now whether extending the UK bombing from Iraq to Syria is likely to reduce, or increase, that threat – and whether it will counter, or spread, the terror campaign Isil is waging in the Middle East.

The prime minister has been unable to explain why extending airstrikes to Syria – which is already being bombed by the US, France, Russia and other powers – will make a significant military impact on a campaign that has so far seen Isil gain territory, such as the cities of Ramadi in Iraq and Palmyra in Syria, as well as lose it.

Crucially, he has failed to convince almost anyone that, even if British participation in the current air campaign were to tip the balance, there are credible ground forces able to take back Isil-held territory.

Last week the prime minister suggested that Kurdish forces or the Free Syrian Army would be able to play that role. He even claimed there is a 70,000-strong force of moderate FSA fighters ready to coordinate on the ground with a western air campaign.

That claim has not stood up to basic scrutiny. Kurdish forces will be of little assistance in the Sunni Arab areas Isil controls. Nor will the FSA – which is now a disparate umbrella group, including elements few would regard as moderate, and mostly operating in other parts of the country. The only ground forces now able to take advantage of a successful bombing campaign are the stronger jihadist and radical Salafist groups.

That’s why the logic of an intensified air campaign is mission creep and western boots on the ground, whatever the prime minister says now about the deployment of British combat troops.

UN security council resolution 2249, passed in the aftermath of the Paris attacks, does not give clear and unambiguous authorisation for UK airstrikes. But it’s a welcome framework, for example, for action by UN member states to cut off funding, oil revenues and arms supplies from Isil territory.

There’s little sign, however, of that happening in earnest. Nor is there yet any serious evidence that it’s being used to coordinate international military or diplomatic strategy in Syria, despite the clear risk of potentially disastrous incidents, such as the shooting down of a Russian military aircraft by Turkish forces.

The prime minister has avoided spelling out to the British people the warnings he has surely been given about the likely impact of British airstrikes in Syria on the threat of terrorist attacks in the UK. And he’s offered no serious assessment of the impact of an intensified air campaign on civilian casualties in Isil-held Syrian territory, or on the wider Syrian refugee crisis.

Most importantly, Cameron has been entirely unable to explain how UK bombing in Syria would contribute to a comprehensive negotiated political settlement of the Syrian war. That is widely understood to be the only way to ensure the defeat of Isil in the country. Isil grew out of the invasion of Iraq, but it has flourished in Syria in the chaos and horror of a multi-front civil war.

Cameron’s approach is bomb first, talk later. But instead of adding British bombs to the others now raining down on Syria, what’s needed is an acceleration of the peace talks in Vienna, involving all the main regional and international powers, with the aim of negotiating a broad-based government in Syria that has the support of the majority of its people. In the context of such a settlement, internationally backed regional forces could help to take back territory from Isil. But its lasting defeat in Syria can only be secured by Syrians themselves.

In the past week I have aimed to give a lead to the growing opposition to Cameron’s bombing plans – in the country, in parliament and in the Labour party. Rejection of 14 years of disastrous wars in the wider Middle East was a key part of the platform on which I was elected Labour leader. However bumpy a ride that has been in parliament, it is essential to learn the lessons of those wars.

In the light of that record of western military interventions, UK bombing of Syria risks yet more of what President Obama called “unintended consequences”.

The prime minister said he wanted a consensus behind the military action he wants to take. He has achieved nothing of the kind. After Iraq, Afghanistan and Libya, MPs thinking of voting for bombing should bear in mind how terrible those consequences can be. Only a negotiated peace settlement can overcome the Isil threat.