Sayyid Imam al-Sharif, who goes by the nom de guerre Dr Fadl, helped bin Laden create al-Qaeda and then led an Islamist insurgency in Egypt in the 1990s.
But in a book written from inside an Egyptian prison, he has launched a frontal attack on al-Qaeda's ideology and the personal failings of bin Laden and particularly his Egyptian deputy, Ayman al-Zawahiri.
Twenty years ago, Dr Fadl became al-Qaeda's intellectual figurehead with a crucial book setting out the rationale for global jihad against the West.
Today, however, he believes the murder of innocent people is both contrary to Islam and a strategic error. "Every drop of blood that was shed or is being shed in Afghanistan and Iraq is the responsibility of bin Laden and Zawahiri and their followers," writes Dr Fadl.