The disclosure was contained in the more than 1,000 pages of previously classified documents about the entertainer's controversial life released under the Freedom of Information Act.

According to one of the files, Frank Sinatra offered his help to the FBI in 1950, as he felt there was an opportunity to do some undercover work for his country's good. However, his offer was turned down.

The same year a confidential federal informant reported that Sinatra had smuggled a million dollars in cash into Italy for mobster Lucky Luciano.

The documents contain a hotchpotch of facts, allegations and rumours about the singer, who died seven months ago.

The BBC's Brian Barron: "No smoking guns in the files of Ol' Blue Eyes"

They date back at least as far as 1943, when the FBI investigated a report that Sinatra dodged being drafted by paying a doctor to classify him as physically unfit to serve in the US military. The allegation was proved to be unfounded.

The files also included a number of threatening letters Sinatra received during his career.

Copies of FBI telegrams noting the threats were among the 1,275 pages along with reproductions of at least one hand-written note from a self-styled "psychic" who believed the singer was bent on dividing the United States "West against East, East against West".

Mobsters

One file from 1971 listed Sinatra along with a virtual who's who of reputed organised crime figures - Aniello Dellacroce, Carlo Gambino and Guiseppo "Joe" Gallo among others - in an alleged plot to extort $100,000 from Ronald Alpert, a former stockbroker.

Sinatra's files will be available at the FBI's Washington HQ

Another section of the papers included a 1955 letter from the special agents in charge of the Philadelphia office to FBI director J Edgar Hoover, reporting that a confidential informant had "advised that ... Frank Sinatra, well-known radio and movie star, is a member of the Communist Party."

Another memo from an agent in Detroit said sources had reported that Sinatra "was never active in the CP or related front group activities in the state of Michigan."

Sinatra, who died in May aged 82, allowed himself to be photographed with mobsters, but always denied any connections with organised crime figures such as Lucky Luciano and Chicago Mafia boss Sam Giancana.

New York columnist Jimmy Breslin, says it was inevitable that Frank Sinatra rubbed shoulders with the mob.

"The man came out of nightclubs. Who runs a nightclub? A professor of western civilisation from Oxford?

"They're wise guys, gangsters, and he sang for them and they were in love with him," he said.

The files were requested after Sinatra's death in May, and before the release, FBI spokesperson Linda Kloss declined to comment on whether they contained evidence of Mafia connections.

"We aren't talking about that. We're just going to let the records speak for themselves," she said.

Although he allowed himself to be photographed with mobsters, Sinatra always denied links with the underworld.

In 1963 he lost his Nevada gambling licence when mobster Sam Giancana was seen in the Cal-Neva Lodge casino, in which Sinatra held a major interest. The license was restored in 1981.