The 2003 invasion of Iraq began a long conflict that cost the lives of 179 British service personnel and more than 100,000 Iraqis, according to several sources.

We were originally promised an independent inquiry back in 2009, to “strengthen the health of our democracy, our diplomacy and our military”.

But five years on – and more than a decade after the war – the final report of Sir John Chilcot’s inquiry remains unpublished.

The man who took Britain to war in 2003 – former prime minister Tony Blair – insisted today that he was not holding up the investigation, contrary to a slew of press stories.

Why has it taken so long for the inquiry to see the light of day?

British soldier secures the scene of a bomb attack east of Basra

What has the inquiry been looking at?

Sir John Chilcot was tasked with looking at Britain’s involvement in Iraq from 2001 to 2009, a period that meant it could address allegations that Tony Blair misled parliament over the case for going to war against Saddam Hussein.

The inquiry is not a court and it does not have the power to say whether anyone involved should face criminal charges. But it will make public criticisms of individuals.

When was the report supposed to come out?

The last public hearing was held in Februrary 2011. At the time Sir John said it would take “some months” for the inquiry to deliver its report, but refused to set out a deadline.

There was talk of a draft report by summer 2012, but since then the timetable has slipped dramatically.

Why the delay?

The inquiry has been negotiating with civil servants in the Cabinet Office for nearly a year over sensitive government documents which it wants to refer to in its report.

Sir John has asked the cabinet secretary, Sir Jeremy Heywood, to clear for publication the records of 200 cabinet discussions, 25 notes from Tony Blair to US President George W Bush and more than 130 conversations between either Mr Blair or his successor Gordon Brown with the president.

In November last year, in the latest official update he has given on the inquiry’s progress, Sir John said: “It is regrettable that the government and the inquiry have not reached a final position on the disclosure of these more difficult categories of document.”

Chairman of The Iraq Inquiry Committee, Chilcot, gestures during a meeting with relatives of British soldiers who died during the war with Iraq

This appears to be the big sticking point. The inquiry doesn’t want to issue a report criticising individuals without knowing what evidence they will be able to publish to back it up.

And it says it can’t complete a process called Maxwellisation – where people who are criticised are warned in advance and given the chance to respond – until the government clears the sensitive material.

Whose fault is it?

We can’t say for sure – but there are a number of suspects.

The Cabinet Office has had nearly a year to come to an agreement with the inquiry on which government papers it can publish and has failed to do so thus far.

Former foreign secretary Lord Owen is among those who have criticised the decision to leave the final word on what can be published to Sir Jeremy Heywood, who was was principal private secretary to Tony Blair when the decisions was taken to go to war.

But all sides say the job of going through thousands of documents is complex and labour-intensive.

The Cabinet Office insists it is working as quickly as possible to declassify documents “subject only to ensuring that national security and foreign policy objectives are not compromised”.

The Americans: the Independent has claimed that officials in the White House and the Department of State are blocking the publication of dialogue between Mr Blair and Mr Bush, citing an unnamed Washington source.

Tony Blair has been widely accused of attempting to slow down the publication of the Chilcot inquiry, a charge he has consistently denied, saying it is in his interests for the report to come out.

Speaking on the BBC’s Today programme, he said: “It certainly isn’t me who is holding it up. The sooner it is published the better from my perspective as it allows me to go and make the arguments.”

It’s not exactly clear how Mr Blair could throw a spanner in the works, even if he wanted to.

We understand that Mr Blair’s co-operation may have been needed if the inquiry was seeking to source private correspondence from him, but the decision on whether to release the last cache of sensitive papers appears now to rest with the government.

Britain's Prime Minister David Cameron and wife Samantha talk with former Prime Minister Tony Blair on the doorstep of 10 Downing Street in London

David Cameron has been accused of being so close to Mr Blair that he has failed to push for the earlier publication of the Chilcot report in a bid to avoid embarrassing his predecessor.

But now Mr Cameron has now taken his strongest line yet on a timetable for publication, with an aide saying: “Both the government and inquiry are keen to proceed to publication as soon as practical. The prime minister very much hopes that can happen this year.”

So when will the report come out?

Despite the current prime minister’s words, he doesn’t have the power to intervene in an independent inquiry and there is still no official timetable.

Even when the wrangling over the government papers is resolved, the process of Maxwellisation will have to start again.

The inquiry has to write to individuals it intends to criticise, allow reasonable time for them to reply, then consider their responses before finalising the report, all of which could take months.

What difference will it make anyway?

A number of other investigations into the circumstances surrounding the invasion of Iraq have already been published, and it could be argued that enough material has been published to inform public opinion.

The 2004 Butler Review identified serious flaws in the intelligence the Blair government used to support its case for invasion was published in 2004.

In the same year, the Hutton Inquiry into the death of Dr David Kelly found that the government had not “sexed up” its dossier on Saddam’s arsenal. Several newspapers called it a whitewash.

The Chilcot inquiry denies its report will be irrelevant, saying it “will address a number of issues of enduring importance in a wide range of areas” and “will also establish a thorough, balanced and reliable account of the run-up to the conflict in Iraq, the military action and its aftermath”.

Attention is likely to focus on conversations between Mr Blair and Mr Bush, which may or may not show that the British government was prepared to back US plans for an invasion before the United Nations voted on it.