Their evidence will kick-start what was already expected to be an explosive few days at the Chilcot inquiry into the war, thanks to the appearance Friday of former prime minister Tony Blair, who led Britain into the conflict.
Michael Wood, the top legal advisor to the Foreign Office at the time, and his then deputy Elizabeth Wilmshurst, who quit her job in protest at the invasion, are both due to give evidence on Tuesday.
According to The Independent on Sunday, Wilmshurst will reveal infighting between officials and ministers over the legality of deposing Iraqi leader Saddam Hussein without explicit United Nations support.
She will say she was far from alone in having doubts about the case and say her boss, Wood, "clearly advised" that the war was illegal under international law, the paper said. He did not make this view public at the time.
A report in The Observer weekly confirmed that Wood was planning to tell the inquiry that the war was illegal because of the absence of a second resolution from the UN Security Council explicitly authorising the use of force in Iraq.
Britain's top legal advisor at the time, attorney general Lord Peter Goldsmith, gave the green light for military action under a UN resolution passed in November 2002, but critics claim he was pressured into it.
Goldsmith is due to give evidence to the Chilcot inquiry on Wednesday.
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