A former head of MI5 today describes the response to the September 11 2001 attacks on the US as a "huge overreaction" and says the invasion of Iraq influenced young men in Britain who turned to terrorism.

In an interview with the Guardian, Stella Rimington calls al-Qaida's attack on the US "another terrorist incident" but not qualitatively different from any others.

"That's not how it struck me. I suppose I'd lived with terrorist events for a good part of my working life and this was as far as I was concerned another one," she says.

In common with Dame Eliza Manningham-Buller, who retired as MI5's director general last year, Rimington, who left 12 years ago, has already made it clear she abhorred "war on terror" rhetoric and the government's abandoned plans to hold terrorism suspects for 42 days without charge.

Today, she goes further by criticising politicians including Jacqui Smith, the home secretary, for trying to outbid each other in their opposition to terrorism and making national security a partisan issue.

It all began, she suggests, with September 11. "National security has become much more of a political issue than it ever was in my day," she says. "Parties are tending to use it as a way of trying to get at the other side. You know, 'We're more tough on terrorism than you are.' I think that's a bad move, quite frankly."

Rimington mentions Guantánamo Bay, the practice of extraordinary rendition, and the invasion of Iraq - three issues which the majority in Britain's security and intelligence establishment opposed privately at the time.

She challenges claims, notably made by Tony Blair, that the war in Iraq was not related to the radicalisation of Muslim youth in Britain.

Asked what impact the war had on the terrorist threat, she replies: "Well, I think all one can do is look at what those people who've been arrested or have left suicide videos say about their motivation. And most of them, as far as I'm aware, say that the war in Iraq played a significant part in persuading them that this is the right course of action to take."

She adds: "So I think you can't write the war in Iraq out of history. If what we're looking at is groups of disaffected young men born in this country who turn to terrorism, then I think to ignore the effect of the war in Iraq is misleading."