I was a Cabinet member in the run-up to the Iraq War – I don't think the Chilcot report will give people what they want

We have waited so long for the Chilcot report that it might be worth pausing to ask what questions it is meant to answer and whether it will heal any of the wounds that the Iraq War opened up in the UK and, indeed, Iraq.

I was a Cabinet member in the run-up to war and responsible for the Department of International Development where we did all we could to prepare for the possible humanitarian consequences of an invasion about which I, like so many others, was deeply sceptical.

It was in June 2009, when the last British combat troops were due to return home, that the then Prime Minister Gordon Brown announced that the inquiry would be set up. He said he was establishing an independent committee of inquiry to enable us to “learn the lessons of the complex and often controversial events of the last six years”.

The inquiry was asked to consider the period between the summer of 2001, before military operations began in March, up to the end of July 2009, when the withdrawal of British troops had been completed. Brown said that the inquiry would be modelled on the Franks inquiry, which looked into the run-up to the Falklands War, but stressed that the Chilcot report would have a much wider remit as it would look at the run-up to the conflict, the war itself and at the reconstruction. The idea was that lessons could be learned in every single area.

Brown concluded that, given the complexity of the issues to be addressed, he had been advised it would take a year. He spelled out that “the committee will not set out to apportion blame or consider issues of civil or criminal liability”.

The aim of the report is to learn lessons from past mistakes, but public expectation will be looking for blame. This clash of expectations may well lead to a sense of disappointment when the report does at last see the light of day.

My view is that lesson-learning is key. Twice now British constitutional arrangements have allowed deep deceit in the run-up to a major war. The first was in the case of Suez in 1956, the second is in Iraq. In both cases, the informality of British constitutional arrangements allowed a Prime Minister to concentrate power in his own hands and deceive the Cabinet, Parliament and country about the reasons for the invasion.

The most important lessons to be learned are about decision-making processes in the UK. Cabinet government in the UK can easily become sofa government and decisions cease to be properly made. The Attorney General is an appointee of the Prime Minister and as we now know he could be lent on to change his advice.

The convention that legal advice must be circulated to the Cabinet can be ignored, as it was in this case, and a massaged legal authority for war presented at the last minute. To prevent another Suez or Iraq, these informal arrangements must be changed.

I am personally willing to accept that Tony Blair believed there were weapons of mass destruction in Iraq. That was the view of the intelligence agencies, although they were clear that the evidence was patchy. Blair certainly grossly exaggerated the evidence and the threat it constituted to justify regime change. This has already been made clear by the Butler report.

Another lesson to be learned – which is also true of Afghanistan and Libya – is the need to make thorough preparations for the aftermath of war. The US was of course the major player in post-conflict reconstruction. Extensive preparations were made in the State Department and quietly by the UN. Then, just months before the date of the invasion, the State Department preparations were junked and a totally new unit established in the Department of Defence to take charge of reconstruction.

In the case of the UK, the decision that we should take lead responsibility in Basra was made very late in the day and the preparations for reconstruction were very limited. There was no acceptance that there would be a need for a further Security Council resolution to authorise reconstruction in occupied territory. There was no reconstruction budget and it wasn't even decided which UK department would lead on reconstruction.

Only the arrangements made by the Department for International Development alongside others to prevent a humanitarian disaster in the aftermath of the invasion held up – to the department's credit, food, water and electricity were supplied across the country.

Let us never forget that there was no immediate threat from Iraq. The date for the invasion was a US choice. The UK could have said it was unwilling to join until preparations were in place. This was not even considered because Blair was determined to stand shoulder to shoulder with the US.

To make sure these mistakes are not made again, it should be made clear that the Attorney General's advice is not independent and the House of Commons must commission its own legal advice. The circulation of planning documents and legal advice to the Cabinet must be made compulsory and governments should be required to give a firm undertaking to parliament that preparations for the aftermath are fully made.

Conclusions such as these, in the face of the tragedies inflicted on Iraq, may seem trivial. But almost everyone in the UK has already reached their own conclusion on Blair’s responsibility; he will live with that forever. If we want to reach a positive conclusion and prevent something similar from happening in the future, we must change our unsafe constitutional arrangements.