He'll provide the voice of Zebedee, a bouncy character on a spring. He joins a cast which includes stars like Robbie Williams and Kylie Minogue.
The original show was on TV in the 1960s, and went on for 12 years, so your parents may well have been fans.
The film, being made by the team behind Chicken Run, should be out in 2005.
A spokesman for Sir Ian said he thought the film would be "fantastic for kids and adults alike".
Robbie and Kylie will play Dougal the shaggy dog and Florence respectively.
As well as old favourites, new characters are being introduced.
Ray Winstone, last seen in King Arthur, will lend his voice to new addition Soldier Sam.
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