The Chilcot report is damning. It exposes a litany of failures over a long period, including reliance on flawed intelligence assessments, lack of planning and insufficient foresight of obvious consequences. But the report also exposes a chilling lack of rigour and a political culture of deference.

Chilcot's indictment of Tony Blair could hardly have been more serious | Richard Norton-Taylor Read more

The passages dealing with legal advice illustrate this in spades. Advice from the then attorney general that military action could be lawful turned on an unevidenced assertion by Tony Blair on 15 March 2003 that Iraq had committed further material breaches of UN resolution 1441. Chilcot describes this assertion as “perfunctory”, dryly observing: “It is unclear what specific grounds Mr Blair relied upon in reaching his view”; a view that was not even formally recorded.

The lack of rigour on this crucial issue is staggering. But so is the lack of challenge. On 17 March 2004, the attorney general told the cabinet it was “plain” that Iraq had failed to comply with its obligation and continued to be in material breach of UN resolutions. Members of the cabinet were not provided with the attorney general’s full advice setting out the conflicting arguments. Had that happened, the cabinet would have been alerted to the fact that between 7 March and 17 March, the attorney general had changed his view on the legal effect of resolution 1441. Questions could then have been asked, although, as Chilcot observes, none of the ministers who later read the attorney general’s earlier advice asked for an explanation as to why his legal view had changed.

Although Chilcot restricts himself to the comment that the circumstances in which it was ultimately decided that there was a legal basis for UK military action were “far from satisfactory”, his finding that “there was no imminent threat from Saddam Hussein” and that “the strategy of containment could have been adapted and continued for some time” reinforces the view I and many others held at the time, namely that the legal case for military action was not made. As I wrote in this paper on 17 March 2003: “flawed advice does not make the unlawful use of force lawful”.

Facebook Twitter Pinterest An undated CIA photograph of laboratory equipment found in a mosque in Iraq after the invasion. Photograph: PA

Chilcot’s analysis of the intelligence assessments tells a similar story. As late as 17 March, Blair was being advised by the chairman of the joint intelligence committee that Iraq possessed chemical and biological weapons, the means to deliver them and the capacity to produce them. These assessments were flawed, but, crucially, as with the legal advice, Chilcot observes, “they were not challenged, and they should have been”.

In response to Chilcot we have already heard the mantra that “lessons will be learned”. No one would disagree. But the prime minister’s assertion that measures taken to change the approach to scrutiny of intelligence and security matters, along with a change of culture, have gone a long way to plugging the gap identified by Chilcot is unconvincing.

In the absence of a written constitution, we still rely far too heavily in the UK on unwritten and unenforceable “constitutional conventions”. That is not good enough in light of the Chilcot report.

What is now needed is a robust and agreed framework, underpinned in law, which sets out the practice and the processes to be followed before the UK engages in military action in the future. A cross-party group should be formed forthwith to take this forward.

A properly evidenced and robust legal basis should be a minimum requirement; as should a fully prepared, realistic and risk-assessed plan. The absence of the former is why I opposed the Iraq war in 2003; the absence of the latter why I voted against military action in Syria in 2015.

No action should be authorised without an informed vote in the Commons

Save for in exceptional circumstances, the full legal advice relied upon in support of military action should be available to all relevant ministers in good time, not on a restricted basis as Chilcot demonstrates happened in 2003. Ministerial oversight of planning and preparation also requires disclosure to the cabinet of a robust, realistic and resourced strategy. That way, frank and informed debate and challenge can take place: debate and challenge that was singularly lacking in the build-up to military action in Iraq.

Beyond that, MPs should be provided with summaries of both the legal advice and the strategy. Then further robust debate and challenge should take place. No action should be authorised without an informed vote in the Commons.

These minimum requirements would not and should not prevent future UK military action. But they would establish a new framework to ensure that any such action had a proper legal and military basis.

They would also begin, finally, to repair the breach caused by the Iraq war. This was, after all, the most divisive British foreign policy decision since Suez. Thirteen years after the invasion, those divisions remain. Hopefully, they can now start to heal and we can start to ensure similar mistakes will not be repeated.