Director Joss Whedon - who will write and helm the sequel - is thought to have already developed a template for the storyline, reported Contactmusic.
"Marvel has a very set agenda of what they want to do, they are already kind of ahead of the game on the sequel because they've got the guy that did it before doing it again and something tells me Joss already has the template for the film," a source said.
A number of cast members will be unavailable to shoot the movie ready for an earlier release date as they will be filming their own stand alone motion pictures.
Robert Downey Jr is to reprise his titular role in 'Iron Man 3' in 2013, Chris Evans will return as the lead character in 'Captain America: The Winter Soldier' in 2014 and Chris Hemsworth is to star in 'Thor: The Dark World' next year.
"In terms of the time frame, I think that it is well in the works. It looks like the release date might be May 1, 2015, but that can change depending on people's schedules," a source said.