A friend of the sixties teen idol revealed Faith worked for the agency to secretly report back on the activities of Fidel Castro.
Faith was approached by a spook at London’s posh Savoy Hotel just before he was due to visit Cuba.
The claims have been made in a new book published by close friend music producer David Courtney.
Courtney recalled how he had gone to meet Faith at the hotel.
He described how Faith “called me to one side and asked me to sit at a table across the way while he continued his meeting.
“I could see he had become more animated with his hands and nodding his head.
“After about 15 minutes he waved over to me to join them. The guy in question was a very pleasant unassuming chap. We talked for a while and he left.
“I said to Adam ‘What was that all about?’ ‘Well,’ he said, ‘I am going to tell you something but you must never repeat it.’
“I agreed. ‘I have been filming a travelogue series for the BBC in Cuba, and that guy is from MI6.
“‘They approached me and said they knew I had built up connections in Havana and asked me to do some work for them there, basically spy for them in preparation for the post-Castro era.’”
Faith died at the age of 62 in 2003.
The meeting was understood to have taken place before a visit in 1997 to the Communist island.
Courtney, who discovered Leo Sayer and also worked with Paul McCartney, Eric Clapton and Jimmy Page, is in the middle of writing his autobiography.
He said that Faith told him he was “crapping himself with fear” about the dictator finding out he had spoken to MI6.
He said that Faith told him about being shown into a room where Castro was seated behind a desk.
Faith said: “He looked up at me and said, ‘I know you.’ He held up a copy of my first record in his hand and said to me in broken English: ‘What do you want if you don’t want money?’”
Courtney said Faith never told him about what he did specifically for MI6 but the music producer is convinced his friend was telling the truth.
Earlier this month there were claims that Richard Whiteley was a spy.
Actor Ricky Tomlinson said Whiteley, best known for presenting afternoon gameshow Countdown, was a spook.
In an interview he said: “Richard Whiteley from Countdown was a member of the intelligence services.”
Whiteley died in 2005. Friends and family disputed the claims.