Twenty-nine previously secret letters between former British Prime Minister Tony Blair and US President George Bush in the lead up to the Iraq war will be made public on Wednesday.
The correspondence will be published as part of the Iraq Inquiry led by Sir John Chilcot and dubbed the "Chilcot report".
The 2.6 million-word report, which is four times the length of War and Peace, has taken seven years to produce.
It was launched in 2009 to discover Britain's role in the Iraq war and was tasked with investigating exactly what led to the 2003 invasion led by the US and the subsequent occupation of Iraq.
The controversial war did not have approval from the UN Security Council and was based on the assumption that Iraqi leader Saddam Hussein had weapons of mass destruction which was later proved to be unfounded.
The letters will make up just some of the hundreds of classified documents set to be released which will show exactly what leaders were thinking behind the scenes.
Parts of them have been redacted, however security sources told The Times this is due to privacy concerns rather than a desire to withhold content.
Origins of the controversial war that highlighted the "special relationship" between US and UK leaders have often been called into question in the UK.
At the time, thousands protested joining the bloody and drawn out conflict that claimed tens of thousands of Iraqi lives. Australia took part in the US led-coalition in the aftermath of the September 11 terror attacks with troops ultimately leaving in 2009.
Mr Blair has been consistently berated for his role in the conflict which is viewed in Britain as misguided at best, criminal at worst.
A report in 2004 found that he exaggerated evidence to MPs, but author Robin Butler said Monday that Blair "really believed" what he was doing was right.
The Chilcot report will not rule on the legality of the invasion, but leaks could shed light on Blair's role in the decision-making on the road to war.
Critics are already lining up against him, with former Scottish first minister Alex Salmond gathering cross-party support for an impeachment or possible legal challenge.
Impeachment is a law that was last used in 1806 and is considered obsolete, but could be revived to put a symbolic mark on Blair's reputation in the history books.
Salmond told Sky News on Sunday that "there has to be a judicial or political reckoning".
Embattled Labour leader Jeremey Corbyn is also thought to be ready to call for Mr Blair to now be investigated for war crimes in what would cap off an explosive 10 days in UK politics.
Blair declined to comment before the report, but has previously expressed regret for the lives lost. However, he said he did not regret removing Saddam Hussein.
Coffins on wheels
The $17.5 million paper took seven years to produce and includes testimony from Blair, his successor Gordon Brown, spy and military chiefs and ministers.
It was delayed by wrangling over what could be published as well as the need to give key figures warning and the right of reply.
It's also a sensitive issue for the families of 179 British personnel who died there with many angry at the poor equipment given to UK troops.
Notable among these were the lightly-armoured Snatch Land Rover vehicles, which were nicknamed "coffins on wheels" for their lack of protection against roadside bombs.
Lawyers representing relatives of 29 British troops that died said they would scrutinise the report for evidence of neglect of duty or misconduct in public office.
This could form the basis of legal action against Blair, his ministers or the government in general, a spokesman for McCue and Partners solicitors told AFP.
The International Criminal Court, which was petitioned at the time to examine possible evidence of war crimes, said on Monday it would consider the report as part of its preliminary examination to determine whether there is a reasonable basis to open an investigation.
However, the legality of the war is outside its jurisdiction.