Sir Christopher Meyer, the former British ambassador to the US, will give evidence today as the Iraq inquiry enters its third day.
Meyer, who was ambassador between 1997 and 2003, was present at many of the key meetings between Tony Blair and George Bush before the invasion of Iraq in 2003.
During Wednesday's session, it emerged that the government was told just days before the invasion began that Saddam Hussein may be unable to use his chemical weapons.
The Foreign Office official Sir William Ehrman told the inquiry that a report suggested such weapons may have been "disassembled".
He revealed that on 10 March "we did...get a report that chemical weapons might have remained disassembled and Saddam hadn't yet ordered their assembly."
He added: "There was also a suggestion that Iraq might lack warheads capable of effective dispersal of agents."
But he noted there was "contradictory intelligence".
"I don't think it invalidated the point about what weapons he had. It was more about their use. Even if they were disassembled the (chemical or biological) agents still existed," he said.
The Liberal Democrats said it was a "dramatic revelation" and contradicted Tony Blair's insistence that Iraq posed a "clear and present danger" to international security.
"This is really damning for the former prime minister," said the party's foreign affairs spokesman Ed Davey.
"To go to war when you are not absolutely certain about your case raises serious questions about whether international law was broken."
Ehrman defended the war on the basis that Saddam Hussein had consistently refused to comply with UN disarmament resolutions.
He said that the Iraqi president failed to meet "two key tests" in UN Security Resolution 1441, requiring him to provide a "full declaration" of Iraq's weapons capacity and to fully co-operate with UN weapons inspectors.
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