We warned Iraq invasion would raise terror attack risk, says ex-MI5 chief

• Lady Manningham-Buller arrives at the Iraq inquiry yesterday; a memo the then deputy head of MI5 sent to the Home Office about a year before the invasion was published yesterday, and we highlight some of the key points she made. Picture: Getty Images

Baroness Manningham-Buller said the Security Service struggled to cope with the volume of plots generated in the aftermath of the invasion in 2003.

Giving evidence to the official inquiry into the conflict, she said ministers had been warned that military action against Iraq would heighten the prospect of attacks by al-Qaeda.

However, she admitted MI5 had been slow to appreciate the main threat would come from "home-grown" terrorists.

Lady Manningham-Buller - the only member of the intelligence agencies to give evidence to the inquiry in public - was scathing about the way intelligence was used to make the case for war.

The evidence of Saddam Hussein's supposed weapons of mass destruction (WMD) had been "fragmentary", she said, and she dismissed Tony Blair's argument that action had been necessary to prevent them falling into the hands of terrorists.

She disclosed MI5 had refused to contribute to the government's dossier on Iraqi WMD in 2002 and said MI5 had assessed as early as that year that Iraqi intelligence agents in the UK would not pose much of a threat in the event of military action against Saddam, right. The agency had, however, warned ministers an invasion would lead to an increased threat from al-Qaeda.

She suggested "a whole generation of young people" had been "radicalised" by what they saw as an attack on Islam, before quickly correcting herself to say: "Not a whole generation, a few among a generation."

She went on: "We were very overburdened with intelligence on a broad scale that was pretty well more than we could cope with in terms of plots, leads to plots and things that we needed to pursue."

Lady Manningham-Buller said she had had to ask Mr Blair for a further doubling of MI5's resources in 2003. "This is unheard of, but he and the Treasury and the chancellor accepted that because I was able to demonstrate the scale of the problem that we were confronted by," she said. "During 2003-4, we realised that the focus was not foreigners. The rising and increasing threat was a threat from British citizens, and that was a very different scenario to, as it were, stopping people coming in."

She dismissed Mr Blair's argument that Britain and the US had needed to take action against Saddam following 9/11 to prevent his supposed chemical and biological weapons being obtained by terrorists. "It's a hypothetical theory.

It certainly wasn't of concern in either the short term or medium term to my colleagues and myself," she said.

Lady Manningham-Buller was critical of the way "fragmentary" intelligence - most of it from the Secret Intelligence Service, MI6 - had been used to make the case for war.

"If you are going to go to war, you need a pretty high threshold … to decide on that and I think there are very few who would argue that the intelligence was not substantial enough on which to make that decision," she said.

MI5 had been asked to contribute to the government's Iraq weapons dossier but had declined. She said: "We were asked to put in some low-grade, small intelligence to it and we refused because we didn't think it was reliable."