The former head of MI5's evidence to the Chilcot panel is a savage indictment of the Blair administration and its advisers

In straightforward, devastating testimony, Eliza Manningham-Buller told the Chilcot inquiry how she had warned about what sensible – but mostly frightened to speak out – senior Whitehall officials believed in 2003: that the invasion of Iraq would increase the terrorist threat to the UK.

More than once, the former head of MI5 emphasised to the Chilcot inquiry that the invasion exacerbated the terrorist threat to the UK and was a "highly significant" factor in how "home-grown" extremists justified their actions.

"Our involvement in Iraq radicalised a few among a generation of young people who saw [it] as an attack upon Islam," she said.

Manningham-Buller said she was therefore not surprised that UK citizens were involved in the 7/7 suicide attacks in London or by the increase in the number of Britons "attracted to the ideology of Osama bin Laden" who saw the invasions of Iraq and Afghanistan as threatening their co-religionists and the Muslim world".

The invasion of Iraq "undoubtedly" increased the terrorist threat in Britain, she said.

There was no evidence of any link between Saddam Hussein and al-Qaida – not even the CIA believed that – Manningham-Buller reminded the inquiry as she pointed to the alternative agenda-driven "intelligence service" set up at the Pentagon by Donald Rumsfeld.

Arguably, she added, it was the US and Britain who, by invading Iraq, "gave Osama bin Laden the Iraqi jihad".

Joint Intelligence Committee assessments warned ministers that an invasion would increase the threat to Britain. If they read them, she said, they would have been in no doubt.

She warned Home Office ministers and officials of the consequences of an invasion but, in a pointed remark, suggested she rarely saw Tony Blair in one-to-one meetings. She added: "The head of SIS [MI6] saw him more frequently than I did".

And what did Sir Richard Dearlove, the head of MI6, tell Blair about the alleged weapons of mass destruction threat posed by Saddam? We still do not know.

Manningham-Buller referred more than once to Sir David Omand, the government's security and intelligence coordinator at the time, who gave evidence to Chilcot earlier this year.

MI6 had "over-promised and under-delivered" when it came to Iraq, Omand said.

She was asked what lessons had been learned from the invasion, and replied: "The danger of over-reliance on fragmentary intelligence in deciding whether or not to go to war. Very few would argue that the intelligence was substantial enough to make that decision."

That is a savage indictment of the Blair administration and its advisers who, unlike Manningham-Buller, kept mum at the time.