In the run-up to war, senior British security and intelligence officials as well as diplomats made it clear that they were strongly opposed to the invasion of Iraq - though not clear enough. Why now, why Iraq, they asked; it would merely increase the terrorist threat, as the joint intelligence committee warned ministers less than a month before British troops and bombers joined the US attack on the country. Concern in Whitehall was shared by some perspicacious Americans, including General Tony Zinni, the former head of US central command, which is responsible for operations throughout the Middle East. He called it the wrong war, fought in the wrong place, at the wrong time.

Now comes fresh evidence that senior British officials tried to persuade the Bush administration to keep off Iraq and concentrate on Afghanistan, the real source of terrorist violence inspired by al-Qaida. On the Brink, the newly published memoirs of Tyler Drumheller - the CIA's chief of clandestine operations in Europe until 2005 - tells of a meeting on September 12 2001. The day after al-Qaida's attacks on America, George Tenet, then CIA director, met three British guests - Sir David Manning, then Tony Blair's foreign policy adviser; Richard Dearlove, then head of MI6; and Eliza Manningham-Buller, then head of MI5. "I hope we can all agree that we should concentrate on Afghanistan and not be tempted to launch any attacks on Iraq," Drumheller quotes the leader of the British delegation as telling Tenet.

In a recent article in the New York Review of Books on Tenet's autobiography, At the Center of the Storm, Thomas Powers points out that Tenet names his British guests but omits what was said at the meeting - while Drumheller reports what was said but was prevented by the CIA (which did not want to upset the British) from identifying who said it.

Powers says the appeal not to attack Iraq came from Manning. Drumheller does not dispute that. In his book he says Tenet responded to Manning by saying: "Absolutely, we all agree on that. Some might want to link the issues, but none of us wants to go that route."

A few days later, a group of diplomats and MI6 officers met their American counterparts at a lunch at the British embassy in Washington. Again MI6 expressed concern that the Bush administration had Iraq in its sights. A senior official (Drumheller, obeying instructions, does not identify the official or his nationality) went further, inquiring what the CIA was going to do once the US had "hit the mercury with the hammer in Afghanistan and the al-Qaida cadre has spread all over the world". The official asked: "Aren't you concerned about the potential destabilising effect on Middle Eastern countries?"

Questioned last week about just how far MI6 and other British officials tried to apply pressure on the Americans, Drumheller told the Guardian: "I think the British did everything they could to keep the US focused on Afghanistan. They understood Iraq much better than we did." One of the things they understood was that there was no link between al-Qaida and Saddam, an assertion made against all the evidence by Dick Cheney and his circle.

The worrying, even terrifying, thing about these and other accounts by former CIA officers is the ease with which America's intelligence agency was swept aside by cliques in the White House and the Pentagon intent on war. The CIA's weakness had a knock-on effect on MI6 as both agencies became victims of the blind determination of their respective political masters.

The Bush administration's obsession with Iraq, and Blair's failure to do anything about it, left a dangerous vacuum in Afghanistan. The Taliban was allowed to fill it, and British soldiers continue to be killed there.

· Richard Norton-Taylor is the Guardian's security affairs editor

richard.norton-taylor@theguardian.com