We can confidently make some assumptions about the Chilcot inquiry, whose report has just been delivered to the Cabinet Office for “national security checks”. It will strongly criticise Tony Blair for promising George Bush that the UK would join the US-led invasion of Iraq in 2003 but keeping parliament and the public in the dark; attack ministers, mandarins and top brass alike for allowing Blair to delay military preparations; and damn the catastrophic failure to prepare for the subsequent occupation of the country.

Will the Chilcot report tell the full story? It’s on a knife edge | Chris Ames Read more

What has received far less attention is the devastating evidence Chilcot heard about the invasion making Britain more vulnerable to terrorism. Blair has always dismissed suggestions that his foreign policy decisions were in any way responsible for encouraging terrorist attacks and “radicalising” young British Muslims as a charge perpetuated by “the left”.

The evidence to Chilcot contradicts his assertion. Lady Manningham-Buller, head of MI5 at the time, bluntly told the inquiry the invasion “undoubtedly increased the threat” of terrorist attacks in Britain.

She said she communicated her view to Blair via Whitehall’s Joint Intelligence Committee (JIC). “The number of plots, the number of leads, the number of people identified, and statements of people as to why they were involved,” all pointed to the increased terrorist threat to the UK.

“Our focus was then on dealing with the manifestations of terrorist threats in the United Kingdom since 9/11. Our work was increasing exponentially. It increased very much more when we went into Iraq … ”

She told the inquiry: “To my mind Iraq, Saddam Hussein had nothing to do with 9/11,” and continued: “Our involvement in Iraq radicalised, for want of a better word, a whole generation of young people. Some British citizens saw our involvement in Iraq as being an attack on Islam.”

Questioned further, the former head of MI5 said: “Although the media has suggested that in July 2005, the attacks on 7/7, we were surprised these were British citizens – that is not the case because really there had been an increasing number of British-born individuals living and brought up in this country, some of them third-generation, who were attracted to the ideology of Osama bin Laden and saw the west’s activities in Iraq and Afghanistan as threatening their fellow religionists and the Muslim world.”

She went on: “I mean, if you take the videos that were retrieved on various occasions after various plots, where terrorists who had expected to be dead explained why they had done what they did, it features. It is part of what we call the single narrative, which is the view of some that everything the west was doing was part of a fundamental hostility to Islam, which predated 9/11, but it was enhanced by those events.”

Facebook Twitter Pinterest Tony Blair on his way from Baghdad to Basra in 2007. Photograph: Stefan Rousseau/PA

Declassified papers released by the inquiry, but which have received little or no attention, reveal that in December 2001, well over a year before the invasion, Sir Mark Allen, head of counter-terrorism in MI6, warned Blair’s aides in Downing Street about the consequences. In a hurried note, he said: “Increased distrust of US motives throughout the Islamic world … terrorists’ motives and grievances reinforced … Anger and resentment in the Arab street. The bombings will be seen as an attack on ordinary Arabs, rather than Saddam.”

As the drumbeat to war echoed around the corridors of Downing Street, others in MI6 disregarded Allen’s warnings, seduced by wildly exaggerated intelligence claims about Iraq’s weapons programme – claims they knew would be welcomed by the government. “There was a sense in which, SIS [the Secret Intelligence Service, commonly known as MI6] over-promised and under-delivered,” Sir David Omand, Blair’s former chief security and intelligence adviser, told Chilcot. “I absolutely agreed with that judgment. It’s precisely what we did,” commented a senior MI6 officer.

We will soon know what significance the Chilcot report will attach to these particularly scandalous features of the run-up to the invasion, with repercussions that are being felt more than 13 years later. One former senior British military commander told me recently he believed the 2003 invasion was the “original sin” that has provoked years of violence, Sunni-Shia sectarian war, and the emergence of Islamic State. Many in Britain’s establishment – Whitehall’s governing class – share that view. They opposed the invasion at the time but kept quiet.

It is not for unelected individuals publicly to oppose elected politicians, the argument goes. In the last major foreign policy disaster – the 1956 Suez crisis – a top civil servant demonstrated his opposition by wearing a black tie every day. The British people as a whole would make their views known at the next general election, he said.

The publication of the Chilcot report, of more than 2 millions words, four times the length of War and Peace, describing events that took place more than a decade ago, must not be the end of the affair. It must be digested and its findings debated. And those held responsible for the most disastrous British foreign and military adventure of recent times – one that has caused more long-lasting damage than Suez – must be made to answer the case against them.

• Chilcot, based on evidence to the inquiry with added material from veterans and victims of the Iraq invasion, edited by Richard Norton-Taylor and Matt Woodhead, will be performed at the Lowry Theatre, Salford, 26-28 May, and at the Battersea Arts Centre, London, 1-10 June