During the invasion, the Duke and duchess fled to Lisbon, Portugal, where they continued to socialise with German agents and sympathisers.

The documentary reveals secret police reports from the Portuguese national archives, detailing the Duke’s visits with representatives of Francoist Spain and his time spent socialising with Ricardo Espirito Santo, a wealthy banker who wrote reports for the Nazis.

In one captured diplomatic cable from the German ambassador to Lisbon, the Duke is alleged to have told Spanish agents that: “continued severe bombing would make England ready for peace”.

The cable also alleges that the Duke believed there would have been peace with Germany had he remained on the throne, and that he was being kept away from England to prevent him boosting “English friends of peace”.

Not long after, Churchill made the Duke the Governor of the Bahamas, removing him from Europe. While travelling to the Caribbean, captured German documents show that Edward sent a coded telegram to Espirito Santo saying that he was willing to come back to Europe.

Dr Lownie argues that this relates to Operation Willie, the German plan to place Edward back on the throne as the head of a puppet state. Traditionally, the Duke had been thought to have been unaware of this plot.