As the foreign affairs select committee has noted, in the space of just a few years Isis has emerged as one of the world’s most notorious and brutal radical jihadi organisations. It has capitalised in the chaos of the Syrian civil war to grow in strength and territory. Following the horrifying attacks in Sousse, Ankara, over Sinai, in Beirut and Paris, no one should be in any doubt that Isis has the capability and intention to carry out further acts of terrorism.

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A strong international response is needed to combat this threat to the peace and security of all of us. And the UK should be part of that response. We are already engaged in military action in Iraq and providing surveillance and logistical support in Syria. The prime minister wants us to go further and outlined his strategy in the House of Commons last week. The test that should be applied to that strategy is whether it is lawful, coherent and compelling.

It would be preferable for the Labour party to come to a view which most MPs support, but this is an issue of such important principle that, if necessary, a free vote should be allowed.

In my view, the military action taken in Iraq in 2003 was not lawful under international law because there was no UN resolution expressly authorising it. The situation now in Syria is different, but calls for great clarity.

The UN resolution passed by the security council on 20 November does not, of itself, provide a legal basis for the prime minister’s proposed course of action. While using some language familiar from other resolutions on the use of force, it does not in fact authorise the use of force. It implicitly supports action that has been, or may be taken, but does not provide a standalone legal basis or authorisation for those actions. That means that the UK and others will have to rely on the varying legal bases that they have been using until now. The prime minister relies on the collective self-defence of Iraq and argues that there is an additional legal basis, namely our own self-defence and that of allies and partners. Without coming to a final view on the prime minister’s additional ground, the collective self-defence of Iraq is, in my view, a sufficient legal basis for the action he proposes.

The question whether the prime minister’s strategy, while lawful, is also coherent and compelling is much more difficult to answer. The argument that there is no logic in taking military action in Iraq but not in Syria because Isis does not recognise the border between them is seductive. But it soon unravels. The situation in Syria is very different to the situation in Iraq. The civil war has a different dynamic; the opposition forces are differently constituted; and, of course, Russia has a heavy involvement in support of the Assad government.

In the circumstances, I do not intend to vote in support of the prime minister’s current strategy

That does not mean that there should be no response; far from it. The UK should put its full diplomatic weight behind the new political talks – the Vienna process. We must work through these discussions to secure a transition to an inclusive government in Syria capable of restoring peace and stability to the country and to the region. We should also use our aid budget to alleviate the immediate humanitarian suffering; improve our contribution to the refugee crisis; and step up our efforts to counter radicalism in the UK.

The question is therefore whether we should go further and engage in airstrikes over Syria. I am not a pacifist and I would back a lawful, coherent and compelling case for the use of military force by the UK against Isis. But, having deliberated carefully on the issue, I am driven to the conclusion that the strategy outlined by the prime minister is flawed.

In my view, airstrikes without an effective ground force are unlikely to make any meaningful contribution to defeating Isis. And there is no effective ground force. If, through international collaboration, a ground force can be agreed, the situation would be different. But the prime minister’s reliance on what he calls “around 70,000 Syrian opposition fighters on the ground who do not belong to extremist groups” to retake the ground from Isis is wholly unrealistic. These fighters are from a disparate number of groups with varying motivations and capabilities. They are also, by definition, opposition fighters and it is difficult to see how the UK and its allies could protect them in retaking ground from Isis without getting drawn into conflict with Russia.

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When pushed on this issue in the House of Commons last week, the prime minister retreated to a suggestion that “the best troops” would be the Syrian army after a political transition in Syria. But that only underscores the central weakness in the prime minister’s case and reinforces my view that the defeat of Isis and the ending of the civil war in Syria are inextricably linked.

In the circumstances, I do not intend to vote in support of the prime minister’s current strategy. I accept that Isis must be defeated and I would be happy to consider a revised strategy. But the current plan is flawed.