The Incredibles director Brad Bird has said he has begun work on the script for the sequel to the hit superhero animated adventure.

Speaking to NPR in the US, Bird said The Incredibles 2 is "percolating", adding that he is "just starting to write" the sequel.

The 57-year-old said that making films is not as easy as it once was.  

"It's very difficult nowadays because everyone has a device with which they can ruin your movie," he said.

"They print things on paper so that nobody can copy them, but they also make them impossible to read when you're on the set."

The original film, which was released in 2004, centred on a family of undercover superheroes who must save the world.