They're four furry creatures with aerials on their heads and TVs in their tummies. They have conquered the nation's toddlers and now they are taking the rave scene and the nation's students, by storm. They are on the brink of being sold overseas. And they have a split on their hands.

All is sweetness and light between the Teletubbies in their show, the children's programme which is rapidly becoming a cult to rival The Magic Roundabout of days gone by. The four cuddly chums, Tinky Winky, Laa Laa, Dipsy and Po, giggle and dance about in the flower-bedecked green fields of Teletubbyland, living on toast and saying "haro" to each other. But in the real world of the actors inside the characters' furry suits, there is discord.

Dave Thompson, the actor who plays Tinky Winky, the purple and tallest one, has been career-plateaued. On the final day of shooting for the new run of the show to be shown this autumn, Thompson, 37, says he received a letter confirming that his "interpretation of the role was not acceptable". He is secretly being replaced.

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"I wasn't given any clues as to what I was doing wrong," he says. "I was officially asked to leave in a letter from an accountant just before everyone else went off to the end-of-shooting wrap party.

"Although I'd already been given the idea that they might let me go, the two directors had told me they were happy with Tinky Winky and I felt I was doing my best."

In retrospect he suspects his voice may have been part of the problem. "The other Teletubbies use their own voices, but mine was dubbed over. At first they asked me to do a high voice and then they changed their minds just before we started filming."

After working hard on 70 episodes for Ragdoll Productions, Thompson was baffled. He believed he had bonded with the other three Tubbies and had enjoyed developing a programme which has gone on to gain a huge and devoted following in a very short space of time. It began at the end of March; yet last week, a magazine survey placed it at number eight in the top ten most-loved children's TV programmes of all time.

The first batch of Teletubbies merchandise will be released on to the market in the next few weeks. We can expect footwear, toiletries, greetings cards and the inevitable figurines.

Although the creatures, invented by Ragdoll's Anne Wood and Andy Davenport, were initially roundly condemned for lacking educational content, their surreal quality has now secured them cult status.

Such is the programme's appeal that security at the secret location of Teletubbyland in Warwickshire has recently become a problem. During The Phoenix music festival last weekend several ravers attempted to break in and tread the hallowed turf. What is more, club DJs have dubbed the soundtrack on to dance mixes and the series is already the subject of a trendy Internet page.

Although he was sad to leave Teletubbyland, Thompson has moved on to greener pastures (if such a thing is imaginable), accepting work as a lion in a lucrative feature film role and going back to his stand-up comedy career. He has, he says, put the incident behind him.

"I am proud of my work for them. I was always the one to test out the limitations of the costume. I was the first to fall off my chair and roll over. I took all the risks."

A BBC spokeswoman would not confirm that the man inside Tinky Winky had been replaced. "We are not allowed to say," she said. "As far as we are concerned they are real."