Spacey was not named directly but police confirmed that the allegation related to the same person as the other two complaints.
Spacey has also been hit with allegations of sexual misconduct and harassment from co-workers and former staffers on the set of TV drama House of Cards.
"On December 13 we received an allegation that the man sexually assaulted a man (Victim 3) in 2005 in Westminster", the Metropolitan Police spokesman said on Wednesday.More news: United States lawmaker proposes ban on Huawei and ZTE
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Star Trek: Discover actor Anthony Rapp who the first to come forward and claim Spacey assaulted him when Rapp was 14. "Officers from the Child Abuse and Sexual Offences Command are investigating".
London's Old Vic theater, located in the Lambeth borough both complaints originated, said it had received 20 complaints of inappropriate behavior regarding Spacey when it asked for people to come forward. Scotland Yard had initiated its primary investigation into those charges soon after.
Similar allegations quickly began flooding in against Spacey, and he was promptly fired from Netflix's House of Cards and Ridley Scott's All the Money in the World. His scenes were reshot with Christopher Plummer replacing him as the billionaire John Paul Getty Sr in the film.