Sylvester Stallone has joined the cast of the upcoming all-star film The Suicide Squad, director James Gunn has revealed.

The movie - due out next summer - is a soft reboot of 2016's Suicide Squad and will star the likes of Margot Robbie, Idris Elba, John Cena, Peter Capaldi and Viola Davis.

Posting a picture of himself alongside Stallone on Instagram, Gunn wrote: "Always love working with my friend @officialslystallone & our work today on #TheSuicideSquad was no exception.

"Despite Sly being an iconic movie star, most people still don't have any idea what an amazing actor this guy is."

Details of Stallone's role are unclear, with principal filming on the movie having already been completed in February before the coronavirus pandemic began.

Confirming the news on his own Instagram account, Stallone, 74, said: "Working with this incredible Director on this astounding project has made this an amazing year. I am a very lucky man to be surrounded by such talent."

The Suicide Squad is currently slated for release on August 6, 2021.