The former attorney general, Lord Goldsmith, will appear before the Chilcot inquiry today, where he is set to face questions about the legality of the Iraq war. The committee will ask Goldsmith whether ministers put pressure on him to agree the conflict was lawful.
Witnesses have already told the inquiry that Lord Goldsmith expressed doubts about whether the war would be legal without a second United Nations resolution.
On Tuesday, Elizabeth Wilmshurst, a Foreign Office lawyer who resigned in protest days before the invasion began, described the legal decision-making process as "lamentable" and lacking in transparency.
She added that it was "extraordinary" that Goldsmith had been asked for his legal opinion about the war just days before British troops went into action.
Former prime minister Tony Blair, who took Britain to war in 2003, is to give evidence on Friday.