One of Britain’s leading experts in international law has said that the Chilcot inquiry into the Iraq war must deliver a convincing account of the mistakes that led to the 2003 conflict to help restore public trust in politics.

The inquiry will publish its 2.6 million-word report on Wednesday amid a tumultuous time in British politics.

Before the report’s release, Philippe Sands QC, author of Lawless World, a book about the Iraq war, said: “Of singular importance is the need for Chilcot to restore trust in the process of decision-making in government.”

Sands said there was a need to establish “precisely what went wrong, why it went wrong and who were the key players in making it go wrong, so that lessons will be learned that will allow us to make sure it never happens again”.

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He added: “If he doesn’t achieve that then the whole thing will have been a waste of time.”

Since it was launched seven years ago there have been fears that the inquiry – overseen by Sir John Chilcot, a former Whitehall mandarin – will pull its punches.

There are suggestions that the report will not accuse former prime minister Tony Blair of misleading parliament, something that would trigger calls for his prosecution. Of crucial importance will be the way the inquiry has treated unpublished material, notably confidential records of conversations between Blair and President George W Bush.

“Has it fairly and accurately summarised the exchanges between Mr Blair and President Bush and the meetings that took place between them?” Sands asked. “That gives us an insight into whether or not the material has been fairly and accurately interpreted by the Chilcot inquiry.”

Sands said he would look closely at the inquiry’s account of what happened at a meeting between Bush and Blair on 31 January 2003. “At that meeting Blair had in his pocket advice from Lord Goldsmith saying ‘You need a second resolution’. We know he then left the meeting with Bush and gave a public statement in which he said nothing had been agreed and spoke in parliament a few days later and basically said ‘Nothing has been agreed’. But I know from the note of the meeting prepared by David Manning [UK ambassador to Washington 2003-07], which is in my book, that recorded at that meeting Bush saying the bombing would begin in March and Blair saying: ‘I am solidly with you, Mr President.’”

The inquiry will not offer a view on whether the war was illegal. Its main remit is to learn the lessons of what went wrong. A key issue will be the analysis of how suspect intelligence was used to justify the invasion of Iraq, chiefly the now-infamous claim that Saddam possessed weapons of mass destruction (WMD). Last year Blair apologised “for the fact the intelligence we received was wrong”. But many experts suspect the intelligence was manipulated to misrepresent Saddam’s capabilities.

In 1995 Hussein Kamel, an Iraqi official who defected, told CIA and British intelligence officers and UN inspectors that, after the 1990-91 Gulf war, Iraq had destroyed all its chemical and biological weapons stocks.

But in the build-up to the invasion, key figures in the Bush administration repeatedly cited Kamel’s testimony as evidence that Iraq possessed WMD. Blair included it in his speech to parliament ahead of the invasion. When pressed in parliament to make Kamel’s testimony public, Blair said the UK did not possess a transcript.

Hans Blix, the UN’s former chief weapons inspector whose team was charged with finding evidence of WMD, has also claimed that the Blair government “misrepresented what we did … in order to get the authorisation [for war] that they shouldn’t have had”.

Blair is expected to challenge claims that experts warned him about the conflict between Sunni and Shia followers following Saddam’s removal.

“The thing I found really shocking when I was researching this was the absence of a plan and a complete failure to make any kind of preparation for the postwar aftermath or even consider what the aftermath might be,” said Steven Kettell, associate professor in politics and international studies at the University of Warwick. “It was criminal negligence on an industrial scale.”

Kettell said the inquiry’s report needed to be explicit in its criticisms. “Otherwise, a significant number of people will say it’s a cover-up by the establishment.”