Cast Members Robert Downey Jr. (R) And Chris Evans Pose At A Press Line
IN PHOTO: Cast members Robert Downey Jr. (R) and Chris Evans pose at a press line for "Avengers: Age of Ultron" during the 2014 Comic-Con International Convention in San Diego, California July 26, 2014. Reuters/Mario Anzuoni

Marvel fans must brace themselves for the biggest crossover ever in Marvel Cinematic Universe. According to a report, Robert Downey Jr will be joining the cast of still-in-works "Captain America 3." The actor will reprise the role of his superhero Tony Stark/Iron Man alongside Chris Evans' Steve Rogers/Captain America.

Variety broke the news on Oct. 13, 2014. According to the report, Iron Man will play a vital role in setting up the Marvel's comic book's Civil War storyline. This will reportedly introduce the new phase of Marvel Cinematic Universe. The news comes hard on the heels of rampant rumours claiming Iron Man may bite the dust in one of the upcoming movies. However, the news shuns off all the speculations as it is noted that Robert Downey Jr is already "in final negotiations" with the moviemakers.

"Civil War" is a Marvel comic book series that was written by Mark Millar and drawn by Steve McNiven. This crossover storyline was launched in 2006-2007 with a tagline "Whose Side Are You On?"

The plot of the third sequel of "Captain America" franchise will see the two Avengers disunited in the wake of their dispute over Superhero Registration Act. This regulation compels superpowered beings to make their powers and true identities known to the government and work as, and in collaboration, with the U.S. police officers.

The report notes that Tony Stark and Steve Rogers do not see eye to eye on this matter and this makes Tony Stark "a villain of sorts in 'Captain America 3.'" The events and the two superheroes' feud will reportedly have "significant effects on 'Avengers 4.'" "Originally, Marvel wanted to hire Downey for a small role, which would have required just three weeks of work," the report states.

However, Robert Downey Jr was looking forward to have a "more substantial role" in the movie. This reportedly infuriated Marvel Entertainment chief Ike Perlmutter, who asked the writers to write Iron Man's character off the script.

The publication reveals that Marvel boss Kevin Feige was keen to make this deal work considering that the "Civil War" storyline could provide the scope for more sequels for the characters in the future. Meanwhile, the sources revealed that Robert Downey Jr will be earning $40 million for his role in "Captain America 3."

The Hollywood Reporter has jotted down the key elements of the "Civil War" story arc in the comics. According to the report, it features a bigger Marvel Universe which brought together Human Torch, Invisible Man, Daredevil, Nick Fury and others as rebellions against the government and Iron Man, Ant-Man, Spider-Man, Black Widow and more who are in favour of the new rule. Furthermore, Tony Stark becomes the villain and supports the government. The storyline also sees teams of superheroes such as The New Avengers, the Mighty Avengers and Secret Avengers. At the end, the report notes Steve Rogers is killed and Tony Stark takes control of S.H.I.E.L.D.

However, it is too early to speculate the movie's plot. At the same time, the movie is yet to be officially named. Badass Digest claims that the film is either going to be titled "Captain America: Civil War" or "Fallen Son." The movie is yet to get a release date. Meanwhile, Marvel has hinted at the launch of a new "Civil War" comic book in 2015.

But before all that, Marvel superheroes will come together once again to fight one of the greatest and darkest supervillains, Ultron, in "Avengers: Age of Ultron." Joining Robert Downey Jr and Chris Evans are Chris Hemsworth as Thor, Mark Ruffalo as The Hulk, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, James Spader as Ultron and more. The movie is scheduled to release on May 1, 2015.