Sylvester Stallone. Picture: AP

Police have confirmed they are investigating Sylvester Stallone over a sexual assault allegation but admit it will be hard to gather evidence.

The 'Rocky' actor was the subject of a police report filed last month by a 50-year-old actress, who claimed the action legend had assaulted her at his office in Santa Monica in 1990.

Officers told The Blast it will be hard to gather evidence because the alleged crime took place so long ago but they are going to "try their best".

The case will be investigated and then presented to the District Attorney for consideration on charges.

Insiders claim the 71-year-old actor did have a relationship with the woman and even took her with him to Israel when he filmed 'Rambo III' but he broke things off when he discovered the unnamed female's estranged husband was very upset by their union, and feared the spouse may have been tied to someone throwing a molotov cocktail into his backyard.

If the investigation concludes the allegations are false, they will change the 'Expendables' star's status to a victim.

And Sylvester's lawyer has already confirmed they will be pushing for the woman to be prosecuted for filing a false report.

The 'Creed' actor can't take any criminal action until the investigation is complete, but he has been told he can pursue civil litigation immediately.

News of the investigation comes a month after the actor's spokesperson Michelle Bega claimed the events categorically "never happened".

She said: "This is a ridiculous, categorically false story. No one was ever aware of this story until it was published today, including Mr. Stallone. At no time was Mr. Stallone ever contacted by authorities or anyone else regarding this matter. It never happened."