Good news for children of all ages: The Simpsons Movie has been awarded a PG certificate, despite a full-frontal image of a naked, skateboarding Bart.

The first details of the summer’s most keenly awaited blockbuster emerged after a screening in London of edited highlights that was surrounded by airport-style security. The invited audience was frisked and forced to surrender mobile phones.

Environmentalism and Christianity are the satirical targets in the big-screen debut of the animated series, which finally reaches cinemas on July 27. Lisa gets a “green-conscious” Irish boyfriend, while Homer’s toxic pollution of Springfield’s river threatens the whole town with evacuation.

There is a “super-special secret guest appearance”, after previous cameos from Tony Blair and Stephen Hawking, which the producers are not yet prepared to reveal. It is unlikely to be President Bush or Simon Cowell, however. “It’s a very popular person,” Matt Groening, The Simpsons’ creator, said.

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The most disturbing image involves Bart eagerly accepting Homer’s dare to skateboard at high speed to Krusty Burger, stark naked. After a series of fortuitous cover-ups, there is a fleeting glimpse of the ten-year-old’s modest, but distinctly yellow, manhood.

Fortunately for the producers, Fox, the British Board of Film Classification has taken a liberal approach. A spokeswoman said: “Natural nudity with no sexual content is acceptable in PG films.” She added: “It will sail over most children’s heads. The Simpsons is really for grown-ups isn’t it?” The film was passed PG for “mild language, innuendo and comic violence”.

However, the naked Bart does leave an indelible impression on his confused contemporary, Ralph Wiggum, who responds: “I like boys now.”In other developments, Grandpa Simpson has an ecstatic seizure during Sunday church and begins speaking in tongues. The congregation attempts to interpret his words but Bart tosses away a copy of the Bible, complaining: “This book doesn’t have any answers.” The Church of England recently encouraged youth workers to use The Simpsons to increase attendances and the Archbishop of Canterbury has declared himself a fan. Introducing the clips alongside Mr Groening, Al Jean, executive producer, said he hoped that the Church would not be offended. “We posit the existence of an extremely active God,” he said. “I don’t think the archbishop will be disappointed.”

There is a guest appearance by the US punk band Green Day, who are stoned by angry fans when they try to talk about the environment. Al Gore’s nose is tweaked when Lisa gives an lecture about pollution, titled “An Irritating Truth”. She finds love with an Irish, rock-loving activist, Colin, who insists he is not Bono’s son. “There is tragedy to the romance because Colin is Irish,” Mr Groening said.

The film marks 20 years since The Simpsons’ television debut and has been years in gestation. Mr Groening said: “We wanted to do things we could not show on television, like a naked Bart. It is also a tribute to hand-drawn animation. This is not a CGI film where you have 1,000 perfect penguins.” Asked what the film was really about, Mr Groening said: “The message is that a man should listen to his wife. But it’s also a romantic film — Homer falls in love with a pig.”

Minnie Driver was among the celebrities who recorded cameo appearances, but her role as a marriage counsellor is believed to have ended on the cutting room floor.

The producers said that the gags had gone through dozens of drafts and the final version had been finished only days ago. Mr Groening promised a DVD in the autumn with scenes deleted from the 87-minute film.

These include a scene involving Homer and Bart and an Egyptian sarcophagus. When Marge berates Homer for letting the boy get trapped inside, Homer says: “He’s got to get over his fear of coffins somehow.” Mr Jean said: “Actually, I’ve stolen that scene for a television episode next season.”

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