A Foreign Office document, which had been classified for six decades and was only recently released by the National Archives, reveals that British diplomats discussed Fidel Castro's "disappearance" with the CIA.

The leaks raise new questions about how much Britain was aware of Washington's covert efforts to eliminate Castro.

How America joined efforts with the UK to kill Castro

Fidel Castro's secret service chief once estimated that 634 attempts on the Cuban leader's life had been made.

Washington's campaign to assassinate Castro remains an infamous example of Cold War covert action, from exploding cigars to poisoned pills.

For the US government, the presence of a revolutionary government in Cuba, just 90 miles off the coast of Florida, was intolerable.

What is less well known is that in the early 1960s, Britain collaborated with Washington to assassinate Castro.

In November 1961, Thomas Brimelow, a high-ranking British diplomat in Washington, met with the CIA alongside his colleague Alan Clark.

Clark, who was visiting the US from his post at the British Embassy in Havana, provided Washington intelligence with a window into Cuba, a country from which the State Department had withdrawn.

Following the opening statements, the CIA men got right to business, asking Clark "whether the disappearance of Fidel Castro himself would have serious repercussions" in Cuba.

Brimelow's minutes from that meeting, marked "secret and personal," show that the British duo did not object to the overt reference to Castro's assassination.

Clark replied by saying that “Raoul Castro had been nominated as Fidel’s successor” and “he might succeed in taking Fidel’s place if adequate time were allowed."

Clark openly added, “If Fidel were to be assassinated, then it was less certain that there would be a smooth takeover. The [state] apparatus, which was apparently strong enough to cope with a gradual changeover, might not cope with a sudden crisis."

The subversive discussion took place months after the White House severed diplomatic ties with Havana and attempted a failed invasion of Cuba's Bay of Pigs.

US President John F. Kennedy authorized Operation Mongoose, a secret program to eliminate Castro by any means necessary, just days after the UK-CIA meeting.

While the United Kingdom was generally sympathetic to the White House's goal of deposing Castro, British officials were reportedly more critical of US strategy, most notably during the botched Bay of Pigs invasion.

The US, UK honeymoon in Havana

However, the British Embassy in Havana continued to share intelligence with Washington on Cuba's military, political, and economic situations, according to declassified documents.

The Pentagon reiterated in March 1962 "how grateful they are for all previous information on the military situation in Cuba."

Months later, the US outlined "priority targets" for Britain's military intelligence gathering in Cuba.

“Almost all of these ‘targets’ are in the Havana area, and they have been selected as they are almost all in areas which members of the Embassy might be able to visit," wrote one British official following a top-secret discussion with the Pentagon.

See more: The 63rd anniversary of the triumph of the Cuban revolution