Jack Straw will give evidence to the Iraq Inquiry amid speculation that he was nursing private doubts about the military action to overthrow Saddam Hussein.
In public Mr Straw, who was Foreign Secretary at the time of the invasion, was one of the most vigorous advocates of the need to disarm the Iraqi dictator of his supposed weapons of mass destruction.
But a series of leaked documents have suggested that behind the scenes he was urging Tony Blair to be cautious about committing British troops to joining the United States in action against Iraq.
In one letter to Mr Blair ahead of his talks with George Bush at the US president's Texas ranch at Crawford in April 2002, Mr Straw warned him that the rewards from his visit would be few while the risks were high.
He said that there was no majority within the Parliamentary Labour Party for military action, and he highlighted the potential legal "elephant traps" warning that regime change was not in itself a justification for war.
He concluded: "We have also to answer the big question - what will this action achieve? There seems to be a larger hole in this than on anything."
At a meeting with Mr Blair and other key ministers and officials in July 2002, he described the case against Iraq as "thin" and said that Saddam's WMD capability was less than that of Libya, North Korea or Iran.
Finally on March 16 2003, two days before the crucial Commons vote on military action, he was reported to have written to Mr Blair advising him to consider alternatives to joining the US-led invasion.
He is reported to have suggested that the prime minister should tell Mr Bush that he was prepared to send British troops once the war was over to help clear up the mess and to keep the peace but to play no part in the overthrow of Saddam.
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However, in his evidence to the inquiry last week, the former Cabinet secretary Lord Turnbull said that he saw no signs that Mr Straw was harbouring any private doubts about the course the Government was pursuing.
