The decision to invade Iraq was made before Tony Blair publicly announced it "but they were looking for a reason" to do it, the head of the Royal Navy at the time has said.
Admiral Lord West, who was first sea lord when the war began in March 2003, said he was told by "some bastard" to ready his forces in preparation for the conflict.
Lord West said: "I think there had been a decision that we were going to invade Iraq, that that was going to happen, but they were looking for a reason to actually do it."
He told The House magazine: "Of course Blair and everyone else will say, 'No, we didn't make the decision until right up to it'. You can always say that, can't you?
"But I would not have told the fleets, the Royal Navy and the Marines, to be ready for war in the northern Gulf by the end of the year. I would not have sailed the Mine Counter-measures Force for the Middle East so they were in place for operations.
"You don't wake up in the morning and think that. Some bastard told me to do it. That's why I did it.
"All the people involved say, 'We hadn't made our decision right up until the UN resolution', and all these other things. Well, you can argue that you never make the final decision till you make your decision. But, actually, they'd bloody decided. That's the reality."
It is not known when Admiral West, who served as a security minister in Gordon Brown's government and is now a Labour peer, was asked to prepare his forces.
He added the findings of Sir John Chilcot's report would be seized on by activists in the Momentum movement that is loyal to Labour leader Jeremy Corbyn.
Sir John is due to publish his long-awaited report on the Iraq War on Wednesday. Admiral West said: "If they find that people did certain things that were wrong, they should be exposed.
"It will be used by Momentum and people like that to say how awful these Blairites are. But I don't think it will do what we wanted it to do, which was to let people really study it to learn the lessons.
"I will be fascinated to see what it says. What one really wants out of it is lessons so that we don't make the mistakes again. I have a horrible feeling what other people want is to be able to get at people. It should be there to learn real lessons."