Edward VIII was bugged by the government during his final days as King, according to official files made public.
Home Secretary Sir John Simon instructed the General Post Office to secretly record the King's telephone calls during the 1936 abdication crisis, papers held in the Cabinet Office for more than seven decades showed.
Wallis Simpson, the King's divorced American mistress, was in France while calls between royal residences and "the continent of Europe" were recorded.
In November 1936, Edward, who had yet to be crowned, told Prime Minister Stanley Baldwin he intended to marry Mrs Simpson. At that time the Church would not remarry a divorcee when their previous partner was still alive.
The King had hoped to survive the crisis but on December 10 1936 he signed the instruments of abdication ending a reign that lasted only 326 days.