MPs are expected to express anger and frustration on Thursday when they debate the extraordinary delay in the publication of the Chilcot report on the invasion of Iraq in 2003.

The inquiry was set up by Gordon Brown (Blair never wanted it) in 2009 when Chilcot expressed the hope its report would be published in 2011.

It has been delayed because Chilcot and his team spent 3 years arguing with successive cabinet secretaries, Whitehall departments, and the intelligence agencies, over what documents the inquiry could publish.

Sir Jeremy Heywood, the cabinet secretary, referred in evidence to MPs on the Commons public administration committee on Tuesday to "disagreements" with government departments on "very sensitive documents".

Back in 2011, in a sharp exchange of letters with Gus O'Donnell, Heywood's predecessor, Chilcot referred to his repeated demands that notes of conversations between Tony Blair and George Bush, in the runup to the invasion, should be published.

"The material requested provides important, and often unique, insights into Mr Blair's thinking and the commitments he made to President Bush, which are not reflected in other papers", Chilcot stressed.

In his third letter in less than a month to O'Donnell, Chilcot wrote: "The question when and how the prime minister made commitments to the US about the UK's involvement in military action in Iraq and subsequent decisions on the UK's continuing involvement, is central to its considerations".

Referring to passages in memoirs, including Blair's autobiography, A Journey, and disclosures made by Jonathan Powell, Blair's chief of staff, and Alastair Campbell, his former head of communications, Chilcot observed bitterly that "individuals may disclose privileged information (without sanction) whilst a committee of privy counsellors established by a former prime minister to review the issues, cannot".

Documents Chilcot said he wanted to publish included Blair's notes to Bush, more than 130 records of conversations between the two leaders, and records of 200 cabinet discussions.

Chilcot and his team also wanted to publish documents that reveal which ministers, legal advisers, and officials, were excluded from discussions on military action. The papers still kept secret include those relating to MI6 and GCHQ.

The content of documents, Whitehall wants to suppress were "vital to the public understanding of the inquiry's conclusions", Chilcot emphasised.

Last summer, Chilcot agreed to a deal whereby a "small number of extracts" or the "gist" of the documents' contents would be published by the inquiry when it publishes its final report. None of the published material would "reflect" Bush's views.

The trouble is Chilcot, a former Whitehall mandarin himself (and member of the Butler review on the use and misuse of intelligence on Iraq) had already agreed that Whitehall would have the final say over what could be published.

The inquiry protocols drawn up in Whitehall and signed off by Gordon Brown said the inquiry would receive "the full cooperation of the government. It will have access to all government papers..." With no hint of irony, they add: The cabinet secretary is writing to departments to underline the need for full transparency".

The protocols read like an invitation to sweep any embarrassing information under the carpet. They refer to the need to avoid releasing any information "likely to cause harm or damage to the public interest...including, but not limited to, national security, defence interests, or international relations.."

Paragraph 15 of the protocols contains this key passage: "Where no agreement is reached about a form in which the information can be published, the inquiry shall not release that information into the public domain. In such circumstances, it would remain open to the inquiry to refer, in its report, to the fact that material it would have wishes to publish has been withheld".

It will be very interesting to see what Chilcot says about this when he finally publishes his report.

Other inquiry chairmen have not been bound by the conditions imposed on Chilcot, and accepted by him. John Major gave Sir Richard Scott the right to publish whatever he considered relevant and necessary, as the secretary of the arms-to-Iraq inquiry noted in a letter to the Guardian last week.

And unlike in Scott, it is the government, not the inquiry, that will determine the date of publication of the Chilcot report.

Chilcot, however, will have have some control over the so-called "Maxwellisation" process that is now finally under way. Blair, Jack Straw, Sir Richard Dearlove, then head of MI6, and many others, are receiving draft passages of the report in which Chilcot intends to criticise them.

The report will not be published until after the general election. Just how long after will depend on how long Chilcot allows those he wants to criticise to defend their corner. That in itself will be telling.

Many of those in senior positions who opposed the invasion say they want to hold their fire until the Chilcot report is published.

These devastating quotes from witnesses to the Chilcot inquiry – and there are many more – should offer a flavour of what we should expect from the report:

• "I suspect if I asked half the cabinet were we at war, they would not have known what I was talking about. There was a lack of political cohesion at the top" – Admiral Lord Boyce, chief of the defence staff at the time of the invasion.

• [The invasion of Iraq] "undoubtedly increased the threat" [of terrorist attacks in Britain] – Lady Manningham-Buller, former head of MI5.

• "I regarded the invasion of Iraq as illegal … The rules of international law on the use of force by states are at the heart of international law. ­Collective security, as opposed to unilateral military action, is a central purpose of the Charter of the United Nations – Elizabeth Wilmshurst, deputy legal adviser to the foreign secretary, then Jack Straw.

• "I was not being sufficiently involved in the meetings and discussions about the [UN] resolution and the policy behind it that were taking place at ministerial level" – Lord Goldsmith, attorney general.

• "There was a sense in which, SIS [the Secret Intelligence Service, commonly known as MI6] overpromised and under-delivered" – Sir David Omand, Blair's chief security and intelligence adviser.

• "I absolutely agreed with that judgment. It's precisely what we did" – senior MI6 officer identified only as SIS2.