The pair is favoured by director Peter Segal for the boxing film, which tells the tale of two rival boxers who are tempted back into the ring for one last bout despite neither of them having fought for years, reported Contactmusic.
Stallone, 64, has appeared in six "Rocky" films while 67-year-old De Niro won an Oscar for his portrayal of Jake La Motta in "Raging Bull".
The comedy, which was first announced in 2010, will see the pair go head-to-head in the ultimate boxing face-off. Doug Ellin, the creator of popular TV series 'Entourage', has been chosen by Warner Bros to write the script.
The film is expected to go on the floors next year. "Stallone and De Niro are expected to come 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 actors are well known for portraying boxers in the past.