Robert Downey Jr is in final negotiations to star in 'Captain America 3'.

The 49-year-old actor is set to reprise his role as Tony Stark/Iron Man in the third instalment of the popular comic book series, which is set for spring 2016 release.

Robert - who will also appear as the superhero in 'The Avengers: Age of Ultron' next summer and subsequently, in 'The Avengers 3' - is said to be on the verge of signing a deal to suit up as the character in the currently untitled Marvel production.

According to Variety.com, Robert's Iron Man is to play a vital role in the action adventure, which has a civil war storyline, and will see his character war against Captain America's alter-ego Steve Rogers, who is played by Chris Evans.

The plot sees the pair clash over the Superhero Registration Act - which forces superheroes to reveal their identifies to the US government - an idea supported by Iron Man but despised by Steve Rogers.

Although Marvel and Robert are yet to confirm the news, the actor recently hinted at reprising his role in a new comic book blockbuster.

He said: ''I know there are going to be a bunch more Marvel movies and they have big ideas of how to do it best and [they're] in the middle of negotiation.''