Britain's National Archives have released records showing then prime minister Winston Churchill's attempts to cover up a Nazi plot to collaborate with members of the British royal family.

The elaborate plan to install the duke of Windsor as king, should the Nazis successfully invade Britain, involved luring the abdicated king out of neutral Portugal to Spain to offer him a deal.

Top secret memos, made available on Thursday, showed how Churchill tried to stall the publication of the plot after World War II as he was worried about how the royal couple would be perceived. Churchill even asked President Dwight Eisenhower in 1953 to delay publication of the memos.

The Nazi plot failed, and the memos were eventually published in 1957 when Churchill was no longer prime minister.