Robert Downey Jr. is concerned about the upcoming remake of comic book classic The Avengers - urging moviemakers to get it right or "it will really suck".

The Hollywood actor has signed on to reprise his role as Iron Man Tony Stark for an appearance in the new film, which is scheduled to hit cinemas in 2011.

The movie will bring together Marvel superheroes Iron Man, Hulk and Captain America.

But Downey Jr. is adamant that producers need to be extra careful when making the picture, as he doesn't want to see the movie flop.

He tells MTV.com, "It has to be the crowning blow of Marvel?s best and brightest because it?s the hardest thing to get right. It?s tough to spin all the plates for one of these characters. If they don't get it right, it's going to really, really suck."

And Downey Jr is convinced the key to The Avengers' success is to keep the movie realistic.

He adds, "The danger you run with colliding all these worlds is (director) Jon (Favreau) was very certain that Iron Man should be set in a very realistic world. Nothing that happened in Iron Man is really outside the realm of possibility. Once you start talking about Valhalla and supersized super soldiers and jolly green giants it warrants much further discussion.?