“It’s going to take time to find them,” Mr Bush said in Lima, Ohio, of Iraq’s supposed chemical and biological weapons as well as a nuclear weapons programme that Washington insisted Saddam Hussein was pursuing.

“But we know he had them, and whether he destroyed them, moved them or hid them, we’re going to find out the truth. And one thing is for certain, Saddam Hussein no longer threatens America with weapons of mass destruction.”

It was the first time that President Bush has raised the possibility that such weapons were destroyed before the war and might not be found.

Since the start of the Iraq war on March 19, US forces have found no conclusive evidence of Iraqi development of chemical or biological weapons, or evidence that Saddam was building a nuclear bomb, but US officials have said they need more time for the search.

The entire UN Security Council debate over whether to use force against Iraq revolved around whether Baghdad possessed weapons of mass destruction.

Saddam’s Government denied any such possession and more than three months of searching by UN weapons inspectors failed to find them.

The United States insisted they were there and Colin Powell, the Secretary of State, went to the United Nations with intelligence information on what banned weapons Washington believed Iraq had.

Many members of the Security Council want Hans Blix’s UN weapons inspectors returned to Iraq to verify Iraqi disarmament, a move rejected by the White House.

Mr Bush said Iraqis with firsthand knowledge of Iraqi weapons programmes included several top officials who had come forward recently. “They are beginning to let us know what the facts were on the ground. And that’s important because the regime spent years hiding and disguising his weapons.” He declined to declare the war over in Iraq, saying US forces still faced danger from a scattered enemy.

In London yesterday Geoff Hoon, the Defence Secretary, told the Today programme on BBC Radio 4 that Saddam had made determined efforts to hide weapons of mass destruction, scattering them around thousands of sites in Iraq, and that it would take some time to uncover them.

He told John Humphrys that they had not been used because of the success of coalition forces right at the outset, disrupting the ability of the leadership of the regime to deliver the instructions required.

“What I’m saying is that the WMD remained hidden as they are hidden to this day, that he did not have the time to recover these weapons from their hiding places, to reassemble the missiles and then to fire them,” Mr Hoon said.

He added that Saddam had been able to fire missiles at Kuwait. “We had no necessary knowledge at the time that those missiles might not have been equipped with chemical warheads. It’s obviously the case now that those weapons were scattered across Iraq, were well hidden; there is now a determined effort under way to locate and identify them.”

Pressed on why Saddam had not used weapons of mass destruction against coalition forces Mr Hoon said: “I believe the reason for that is because military action followed fairly quickly on the end of the weapons inspections programmes, and having hidden away those weapons having dismantled missiles, having scattered them to the far corners of Iraq . . . it then was extremely difficult for him in time to be able to reassemble them, not least because he was well aware we were watching very carefully . . . and that would obviously have indicated the fact that he had such weapons and was able to use them.”

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