When the Duke and Duchess of Windsor sailed into Palm Beach, Florida on the SS Berkshire from Nassau on April 18, 1941, they were looking forward to enjoying three days of relaxation, playing golf, drinking and gossiping with American high society.

But unbeknownst to this infamous couple, the night before the ship docked, President Roosevelt had instructed FBI chief J. Edgar Hoover to launch what was to be an extraordinary covert intelligence exercise, one that had to fool both the exiled royals and the American secret service agents guarding them. The final FBI report, recently released under the U.S. Freedom of Information Act, running to 227 pages, reveals that the Duke and Duchess of Windsor harboured pro-Nazi sympathies and that Edward was earmarked by Hitler as a potential puppet king if Hitler defeated Britain.

There has long been a great deal of speculation about the Windsors and their political views, which has always been vehemently denied by the ex-king, the establishment and Philip Ziegler, the official biographer. But there is more than the sole FBI report that is damning. Recently, a number of key documents have been released, as well as British military intelligence reports and, crucially, the sensational publication of the diaries of Sir Alan Lascelles, the Duke's private secretary.

Together, these prove that Edward VIII's sympathies lay squarely with Hitler's regime. Now, for the first time in The Nazi King, the full inside story is told, in which Edward and his wife were warmly welcomed by Hitler in Germany and thereafter Edward passed on top secret Allied information to the enemy. It's a tale of treachery, corruption and depravity at the heart of the British monarchy - and one that could have changed the course of history.