Robert Downey Jr is rooting for Scarlett Johansson to lead her own standalone Black Widow movie.

The actress has appeared as Natasha Romanoff aka the Black Widow in Iron Man 2, Captain America: The Winter Soldier, Avengers Assemble and will reprise the role in Avengers: Age Of Ultron.

There has been an ongoing discussion among fans that there need to be more female superheroes - not just sidekicks - in the Marvel movies, and writer Stan Lee recently hinted that a Black Widow spin-off movie could be in the works.

Asked by USA Today if he thought the Black Widow should front her own film, Iron Man star Robert said: "I think that the interesting thing particularly after Guardians with Zoe (Saldana)... even from the first Iron Man where Pepper was kind of this really... to me the Iron Man franchise would never have taken off without (Gwyneth) Paltrow.

"There's something about her that grounded the story. She's not your typical lady in a superhero movie, and then by Iron Man 3 she's swallowing serums and putting on suits and kicking (butt) and all that stuff."

He added: "It would be kind of more appropriate for a character that already was like a Black Widow (to lead a movie). It just seems like whatever Scarlett does people want to go see it."

But the Iron Man star added that he didn't think it was just the female characters who deserved their own movies.

He said: "The funny thing is honestly at this point everyone deserves a franchise. I think Jeremy Renner [Hawkeye] is - when folks see the Avengers: Age Of Ultron he's just a rock star, a badass.

"And [Mark] Ruffalo [The Incredible Hulk] is pumped. He does great (work). I'd like to hear them talk even more seriously about a Hulk franchise, because that's been one of the toughest ones to get right. But I'm sure that my parent company is feeling expansive and and bold after the summer they've had."