The Public Record Office documents confirm for the first time what historians have long suspected - that Edward, the Duke of Windsor, was a firm Nazi sympathiser and his American wife was a malign influence.
A memorandum released by the Foreign Office, 60 years after the abdication, provides the most startling evidence yet of the Windsors' willingness to collaborate with Hitler.
The couple had left Britain for neutral Portugal, but there were government concerns throughout the summer of 1940 that the Nazis might take Edward - by force or persuasion - with the intention of installing him as a puppet king in the event of an invasion.
The latest evidence suggests that force, at least on the part of the duchess, might not have been necessary. Dated 7 July 1940, it comes in the form of a memorandum from an informant inside occupied Czechoslovakia to Sir Alexander Cadogan, permanent secretary at the Foreign Office. It says: "A new source in close touch with Von Neurath's [the German protector of Bohemia's] entourage in Prague has reported that the Germans expect assistance from the Duke and Duchess of Windsor, the latter desiring at any price to become Queen. The Germans have been negotiating with her since June 27.
"The status quo in England expect an understanding to form an anti-Russian alliance.
"The Germans propose to form an opposition government with the Duke of Windsor, having first changed public opinion by propaganda. The Germans think King George will abdicate during the attack on London."
Royal historians were not surprised by the contents of many of yesterday's papers - particularly those once again demonstrating the duke's pro-Nazi sympathies and his embarrassing comments and behaviour after being sent to act as Governor of the Bahamas in 1940. But the confirmation of Wallis Simpson's role in the affair was regarded as significant.
"This dots the i's and crosses the t's," said Harold Brooks-Baker, publishing director of Burke's Peerage.
"She was always regarded as bad news, particularly by the Americans. Roosevelt was always worried about Edward's Nazi leanings and considered her the best thing that ever happened - she stopped him having to deal with a pro-Nazi king."
The revelations also go some way to explaining the Queen Mother's lifelong loathing of the duchess.
There are many more references, however, to the duke's aspirations and to his Nazi leanings.
One intelligence report from a Spaniard, Count Nava de Tajo, to British diplomats said that the duke believed there could be a revolution in Britain, resulting in the abdication of King George VI. He then hoped that a subsequent Labour government would invite him back to take the throne.
Later, a senior Foreign Office specialist covering America wrote to Sir David Douglas-Scott, assistant under-secretary of state, complaining about an interview the duke had given to an American newspaper arguing against US involvement in the war.
Clearly furious, he concludes: "I propose that he now be told of the harm that he has done, and strictly prohibited from giving any more interviews at all without having his texts vetted and authorised at home... [This] may help to correct the gaffe of sending him to this post. It - or any post near the USA - should have been the last chosen."
Duke and the Nazis, page 6