John Lennon was dismissed as a Communist threat to the US only because he was always stoned, secret FBI papers revealed yesterday.

The ex-Beatles singer was thought to be a ringleader of revolutionaries plotting to hijack a Republican conference, the documents show.

But his drug-taking - which in the early 1970s included heroin, cocaine and marijuana - ultimately ruled him out of FBI inquiries.

Marilyn Monroe, Lucille Ball and Albert Einstein were also investigated as suspected Communists, according to the files, published for the first time.

From the 1950s to the 1970s, the FBI kept many celebrities under close watch for links to radical politics, the mob, gambling, drink or drug abuse.

After seeming undecided about radicalism in his 1968 Beatles song Revolution, Lennon became more politically active after moving to New York in the early 1970s.

He hung out with activists such as Jerry Rubin and Abbey Hoffman, and recorded songs and joined marches for jailed drug offender John Sinclair and 'Black Panther' supporter Angela Davis. He also complained about having his phone tapped, as he struggled to win a US green card despite a British drug conviction.

Lennon was being watched shortly before the Republican National Covention of 1972, when Richard Nixon was approved as presidential candidate.

The ex-Beatle was hoping to extend his travel visa so he could 'engage in disruptive activities surrounding RNC', an agent claimed in the reports.

But while Lennon 'appears to be radically orientated', he 'does not give the impression he is a true revolutionist, since he is constantly under the influence of narcotics', the report says.

Monroe was investigated after rumours she might have applied for a visa to visit Russia.

Einstein was ruled out as a Communist but alarmed FBI agents because of his younger, more suspicious associates.

The Doors were investigated because so many people wrote to the FBI complaining about the band's 'filthy and vulgar' music.