"The Avengers" in 2012 became the movie with the third biggest box office in history, raking in more than $1.5 billion, but Marvel Studios is not sure that its actors, including Robert Downey Jr., will return to shoot "The Avengers 2".

Downey, who is not under contract for a sequel, is the chief concern.

The actor earned more than $50 million for "The Avengers", according to Deadline, and in fact it could be as much as $80 million, an amount similar to what he will bank following the success of "Iron Man 3", almost certain to top $1 billion in ticket sales.

According to The Hollywood Reporter, the 48-year-old Downey took time at a big dinner of well-wishers at Spago restaurant in Beverly Hills May 6 to talk with studio execs, in an attempt to establish the basis of a new contract covering Phases 2 and 3 of "The Avengers".

Such franchises as Batman, Superman, Spider-Man and even James Bond have have remained popular even when the actors playing them have been switched, but "The Avengers" was the philosopher's stone of Marvel's success and the studio naturally wants to give it continuity.

"The Avengers 2", whose production is scheduled for next February, has an expected release date of May 1, 2014, while "The Avengers 3" will probably screen in 2017.