Secret correspondence between the exiled Duke and Duchess of Windsor and their confidant Kenneth de Courcy has revealed a dastardly scheme to change the course of British history by denying Queen Elizabeth II the crown, says royal biographer Christopher Wilson.

It was the spring of 1946. The Second World War had drawn to a close, King George VI’s health was starting to fail and, from their homes in Paris and the south of France, the exiled Duke and Duchess of Windsor were having deeply ambitious thoughts.

More than 60 years ago, according to correspondence unearthed in a Californian library, the former King Edward VIII considered the idea of returning to Britain to become Regent, pushing aside his niece – now the Queen.

The natural successor, and heir apparent, was the then Princess Elizabeth. But in the spring of 1949, when the plot was at its height, she was just 23 – four years younger than Prince William is today – and at the time there was a heavy bias against her taking the throne so young because she was perceived to be vulnerable to “the Mountbatten influence” – a reference to the combined forces of the dynastically ambitious Earl Mountbatten, and his nephew Prince Philip of Greece, now the Duke of Edinburgh, whom the Queen had married in 1947.