London, April 10 : 'High School Musical 3' is yet to hit theatres, and Disney Channel has already started working on the fourth instalment, it has emerged.

Disney Channel Worldwide president Rich Ross told the Hollywood Reporter that 'High School Musical 4' is under pre-production.

However, it is not confirmed whether the film's biggest stars Zac Efron, Ashley Tisdale and Vanessa Hudgens will return for the fourth instalment.

"Are we going to have all the cast back? Probably not... Will we have some of them? I hope so," BBC quoted Ross, as saying.

While 'High School Musical 3: Senior Year' will open in cinemas, Ross said the fourth sequel was 'likely' to be a TV movie.

When High School Musical debuted on the Disney channel in January 2006, it took the US by storm and went on to become a worldwide hit.

The second TV movie shattered cable TV records last summer with a premiere watched by 18.6 million viewers and it has been a hit internationally as well.

It boosted Disney Channel to the No. 1 position among cable networks in primetime in 2007.

Production of part three will kick off in Utah later this month and it is due to hit cinemas in October.

ANI