The 'Rocky' legend has signed up to play a retired fighter, who decides to re-enter the boxing ring and face off against De Niro's alter-ego in the upcoming Warner Bros action comedy, reported Deadline.
"Stallone and De Niro are on board as they're both fans of the director and have both sent themselves up in comedies in recent years. If they go head-to-head in the ring it will certainly be a little piece of Hollywood history," a source said.
Both stars are well known for portraying boxers in the past - Sylvester, 66, has appeared in six "Rocky" movies and 69-year-old De Niro won an Oscar for his portrayal of Jake La Motta in "Raging Bull".
Rumours of the film have circulated since October 2010, but it has officially been given the go-ahead after the leading men successfully re-negotiated their roles.
Segal will be directing and Doug Ellin, creator of hit TV show 'Entourage', is currently working on a re-draft of the script.
Kevin Hart, who starred in "Think Like A Man" and "The Five Year Engagement" this year, is on board to play the fight promoter who gets the retired boxers back in the ring for the ultimate face-off.
The film is expected to being production in 2013.
