Bart Simpson used to be an underachiever and proud of it. These days, it seems, he’s an Operating Thetan VII in the Church of Scientology and proud of that, too.

At least, that is what Scientologists were led to believe this week when they received an automated telephone message featuring the voice of Bart inviting them to the Scientology Flag World Tour, an event being held in Hollywood tomorrow.

The message was recorded by Nancy Cartwright, the 51-year-old voice actress and high-ranking Scientologist who has played Bart in The Simpsons since the show was created in 1987. “Yo, what’s happenin’ man,” begins the message. “This is Bart Simpson. Haha! Just kidding, don’t hang up, this is Nancy Cartwright.”

Ms Cartwright moves between her own voice and that of Bart for the rest of the message. “It’s gonna be a blast, man,” she promises of the event, urging listeners to call and confirm their attendance.

When the message leaked out on the internet yesterday, Ms Cartwright’s bosses at The Simpsons had — as Bart might say — a cow. Although the details of the actor’s contract are not known, it is thought that the copyright of Bart’s voice is owned by Twentieth Century Fox, part of News Corporation, parent company of The Times, and therefore cannot be used without permission.

In a public statement the show’s executive producer, Al Jean, attempted to make light of it. “The Simpsons does not, and never has, endorsed any religion, philosophy or system of beliefs any more profound than Butterfinger bars,” he said.

Ms Cartwright, who earns an estimated $400,000 (£280,000) per episode, was recently awarded Scientology’s Patron Laureate Award after reportedly donating $10 million to the organisation, founded in the 1950s by L. Ron Hubbard, the science-fiction writer. As an Operating Thetan VII — and therefore an individual who can operate independently of her body, according to the movement’s website — she has attained the second-highest rank in the Church.

It is not the first time that her association with Scientology has created headlines. Two years ago it was reported that she objected to a veiled joke in the show’s script about Scientology — it is thought that the offending line described Mormonism as “the second freakiest religion in America” — and that as a result the wording was changed. At the time, Ms Cartwright’s publicist said: “Scientology was never mentioned, and to infer that it was is to try and make a story where there isn’t one.”

Other shows have also had their problems with Scientologist actors. The late Isaac Hayes left South Park after objecting to an episode that mocked both Scientology and Tom Cruise, perhaps Hollywood’s best-known follower of the belief system. It is thought that Cruise later put pressure on Viacom, the parent company of Comedy Central, which broadcasts South Park, to stop the episode being rerun.

The head of Viacom, Sumner Redstone, subsequently declined to renew Cruise’s movie production deal with Paramount Pictures. “His recent conduct has not been acceptable,” he said.

Critics accuse the Church of Scientology of being secretive, profit-motivated and cult-like. In the 1970s, a series of FBI raids found that Scientologists had infiltrated and wiretapped the Internal Revenue Service and other government agencies; several members, including Mary, Hubbard’s wife, were jailed. The organisation says that it has reformed itself.

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