Disney has begun work on a fourth installment of the High School Musical film series.

High School Musical 3: Senior Year is set for release later this year and fans were led to believe the movie would be the final episode in the hit franchise.

But now the company has revealed plans are well underway for another film, with a script already being penned.

Disney Channel Worldwide President Rich Ross confirmed the news to MTV.com, saying, "We are writing High School Musical 4.

"We have learned that just putting a number next to a title is a very old idea. We have to have a new story."

All the stars of the original TV movie, including Zac Efron, Vanessa Hudgens and Ashley Tisdale, signed up for the third picture.

But Ross fears Disney might not be able to persuade all of the original cast to reprise their roles for a fourth time.

He adds, "Are we going to have all the cast back? Probably not. Will we have some of them? I hope so."