Tony Blair and his ministers went to war in Iraq despite repeated warnings from senior Government advisers that they had “no leg to stand on” legally.

During evidence to the Chilcot Inquiry into the war against Saddam, it emerged that Foreign Office lawyers were “unanimous” in their view that going to war without a United Nations mandate would be a “crime of aggression” likely to damage Britain’s standing in the world.

Sir Michael Wood, chief legal adviser at the Foreign Office, painted a vivid picture of how his team repeatedly intervened privately to correct ministers who were stating that a fresh UN resolution was not legally necessary, but their advice was ignored.

A secret letter was dramatically declassified midway through the hearings showed that Jack Straw, the then-foreign secretary, told his chief legal adviser that he was being “dogmatic” by warning that war would be illegal.

In what was described as a highly unusual move, Mr Straw wrote to Sir Michael, a renowned expert in international law, boasting that while home secretary he had ignored official legal advice “time and time again”.

The inquiry also heard that Lord Goldsmith, the then-Attorney General, changed his mind over the space of a week about the legality of the war after being ordered during a meeting chaired by Tony Blair to “reflect further”.