Tony Blair said he was convinced by the intelligence reports he was receiving that Saddam Hussein did have weapons of mass destruction.
Giving evidence to the Iraq Inquiry, the former prime minister acknowledged some of the reports he was given warned some of the intelligence was "sporadic and patchy".
But throughout the build-up to the invasion, he said advice from the Joint Intelligence Committee (JIC) - the UK's most senior intelligence body - was that Saddam was continuing his WMD programmes.
"It is hard to come to any other conclusion than that this person is continuing WMD programmes," he said.
"When you are the prime minister and the Joint Intelligence Committee is giving you this information, you have got to rely on the people doing it, with the experience and with the commitment and integrity as they do.
"Of course now, with the benefit of hindsight, we look back on the situation differently.
He strongly defended his claim in the Government's Iraq dossier, published in September 2002, that the intelligence had established "beyond doubt" that Iraq had WMD.
"What I said in the foreword was that I believed it was beyond doubt. I did believe it and I did believe that it was beyond doubt," he said.
He accepted the dossier should have made clear the now notorious claim that Iraq had WMD which could be launched in 45 minutes referred to battlefield weapons and not long-range missiles.
"It would have been better to have corrected it in the light of the significance it later took on," he said.
With hindsight, he said the Government would have been better off just releasing the straight JIC reports rather than compiling the dossier.
"Now, I would take government out of it," he said.