The disclosures come in the new edition of "Lawless World" by Phillipe Sands, professor of international law at University College London.
The most controversial evidence in the book revolves around the minutes of a meeting between the two leaders - at which professor Sands also alleges that president Bush proposed a plan to try to provoke Iraq into war. The idea was to paint and American U2 spy plane in UN colours and fly it over Iraq in the hope that it would be shot at.
Opposition MPs in Britain have said the book is further evidence of a "rush to war" on the part of the Blair government. Professor Sands has been telling the World Today more about the meeting at the heart of his new revelations.
Downing Street has refused to comment on the authenticity of the source material for the book. But they have issued this statement: "the prime minister only committed forces to Iraq after securing the approval of the House in the vote of March 18th 2003".