Tony Blair's reputation will be seriously damaged by the upcoming Chilcot Inquiry into the Iraq War, according to a senior source who has discussed the report with its authors

Tony Blair's reputation will be seriously damaged by the upcoming Chilcot Inquiry into the Iraq War, according to a senior source who has discussed the report with its authors.

The report - which will finally be published on July 6 - will also deliver an 'absolute brutal' verdict on the former Labour prime minister, ex-foreign secretary Jack Straw and the former MI6 boss Sir Richard Dearlove, a former government minister added.

Mr Blair 'won't be let off the hook' over claims he offered British military assistance to US President George Bush before the invasion of Iraq in 2003.

But some of the harshest criticism will be reserved for Mr Straw over the aftermath of the war and the top general who oversaw the city of Basra after it was captured, a source told the Sunday Times.

Mr Straw has been sent a 5,000-word letter by the Chilcot inquiry listing alleged failings over the Iraq war, sources said yesterday.

The length and weight of detail of potential criticisms indicate the report may draw tough conclusions on the war’s architects.

Mr Straw, who became Foreign Secretary in 2001, is understood to be at risk of criticism over his handling of the run-up to conflict in 2003 and his department’s role in the British occupation of Basra afterwards.

The former minister told the Sunday Times that British forces 'did make a mess of the aftermath' and will describe the 2007 withdrawal from Basra as 'embarrassing'.

'Serious mistakes' were made by senior generals running Basra and other southern provinces in the country, the report will say and misjudgments were so bad that British troops 'had to be rescued by the Americans'.

The aftermath of the 2003 invasion is expected to be covered at greater length in the report than the build up, but public attention will be much more focussed on the decisions and the period before British forces joined American troops to invade Iraq in 2003 in search of Saddam Hussein's weapons of mass destruction.

Mr Dearlove, who served as the head of MI6 at the time of the invasion, will be criticised for failing to stop Blair's government from putting a 'gloss' on the intelligence surrounding Saddam's apparent stock of weapons.

It led to a British government document - which became known as the 'dodgy dossier' - published in September 2002 that claimed the Iraqi dictator could attack British targets within just 45 minutes.

'The intelligence community should have resisted' the claims in the memo, the Chilcot report will reportedly say.

Remarkably, the source close to the report's findings said Mr Blair's cabinet did not have 'the full picture' - suggesting his informal 'sofa style' of government led to mistakes and oversights in the run up to the 2003 invasion.

Tony Blair (pictured with George Bush in February 2001 at Camp David) 'won't be let off the hook' over claims he offered British military assistance to US President George Bush before the invasion of Iraq in 2003, a source close to the Chilcot report said

Brutal verdict: The report is set to deliver an 'absolutely brutal' verdict on figures including Jack Straw (left) and former MI6 boss Sir Richard Dearlove

On Mr Blair's willingness to send in British troops to support the Americans, the source said the report will make it 'clear that he did commit himself to Bush at an early stage and didn't want to be seen as letting Bush down'.

A former minister with knowledge of the panel discussions said: 'It will be absolutely brutal for Jack Straw. It will damage the reputation of a number of people, Richard Dearlove as well as Tony Blair and others.'

In a damning verdict on the aftermath, the source said: 'We sent in inexperienced people. People were put in positions where they couldn't succeed. We didn't quite know what we were doing. After the invasion we found it very much more difficult than we had expected.'

It will also reopen the debate about Britain and America's responsibility for enabling the creation of ISIS.

Sir John Chilcot's report will finally be published on July 6 - more than seven years after it was first ordered by Gordon Brown in 2009 and is 2.6 million words long - four times the length of Leo Tolstoy's War and Peace.

But there are fears that the length of the report will make it 'unfocused' as the blame will be at so many doors.

Further questions will be raised over the special relationship with America where British diplomats in Washington were kept in the dark over post-invasion plans.

Mr Blair and Mr Straw declined to comment on today's revelations.

Sir John Chilcot's report will finally be published on Wednesday July 6 some seven years after it was first ordered by Gordon Brown

The inquiry was set up by then-Prime Minister Gordon Brown in 2009 and examined the events from the summer of 2001, through the 2003 invasion, and until the withdrawal of British combat troops in 2009.

The inquiry took evidence from more than 150 witnesses, holding more than 130 sessions of oral evidence.

In addition, more than 150,000 government documents were scrutinised by the inquiry - including many previously classified papers.

The inquiry was tasked with identifying the decision taken across the crucial period and identifying and lessons to be learned from them.

The date of July 6 is likely to cause intense frustration among those who believe the report should have been published years ago.

One of the key reasons for the continued delays was thought to be the so-called Maxwellisation process - the legal practice where individuals due to be criticised in an official report are sent details of the criticism in advance to give them a right to reply before publication.

Further delays were confirmed by David Cameron last month after it emerged that spy chiefs had been drafted in to vet the findings.

It meant the report would not be published before the June 23 EU referendum. Government officials were keen for the report to be published after the vote to ensure it did not influence the outcome.

The long list of delays have heightened concerns that the final findings will be a 'whitewash' as key details could have been removed.

Reg Keys, whose 20-year-old son Lance Corporal Thomas Keys died in an ambush in Iraq in 2003, said an independent official must oversee the vetting process to ensure was not a 'whitewash'.

He told the Telegraph: 'There needs to be a referee almost – if someone says 'I am taking this out it needs to be shown to an independent person' otherwise it will be a whitewash, it will be sanitised.'