Reese Witherspoon and Scarlett Johansson have joined the cast of animated comedy 'Sing'.

Reese Witherspoon

Reese Witherspoon

The two actresses will voice characters in the new movie from Universal and Illumination Entertainment, joining Matthew McConaughey, Seth MacFarlane, Taron Egerton and Tori Kelly.

Matthew will play a koala, Buster Moon, who decides to stage a singing competition at his struggling theatre.

He explained: "Buster is a salesman and a survivor, eternally optimistic and someone who will do anything to make sure 'the show goes on."

Reese will play a frazzled mother of 25 piglets, who gave up her dreams of singing success when she was younger.

The actress told USA Today: "She rarely has a second to herself, so she finds her moments of freedom intersect with this passion for music. She embodies and embraces that singer in all of us who just belts it out when no one is looking."

Scarlett's punk-rock porcupine will have trouble with her boyfriend when she makes it through to the competition but he doesn't.

She said: "Ash is an artist at heart who discovers her true voice with the support of her friends and fellow artists. In fact, she represents a lot of teenagers going through the same experience of embracing their true selves as they come of age."

The film will feature more than 85 songs and the script has been written by Garth Jennings, who also directs.