LOS ANGELES (Reuters) - Elvis Presley and The Beatles top the list of most-forged celebrity signatures in 2012, with less than half of their autographs for sale certified as genuine, memorabilia authenticators PSA/DNA said on Thursday.

The King and The Fab Four British rockers, who topped the list two years ago when it was last released, joined notable figures such as former U.S. President John F. Kennedy and late pop star Michael Jackson on the list of most-forged celebrity signatures.

Late American astronaut Neil Armstrong landed at No. 3 on the list, after fake Armstrong signatures rose significantly after his death in July.

One reason forgeries of Armstrong's autograph soared was that he rarely signed for fans during his life, Joe Orlando, president of Newport Beach-based PSA/DNA, told Reuters.

"Armstrong is someone who is very conscious of the value of his own autograph," Orlando said. "Even before he passed away he was very tough to get...It really heightens the level of his market."

Secretaries and assistants responding to huge volumes of fan mail are one reason for fake signatures floating through the marketplace, said Margaret Barrett, director of entertainment and music memorabilia at Heritage Auctions in Los Angeles.

"Back in the day, the kids would write to the movie studios," Barrett said.

"There was absolutely no financial gain 50 years ago and secretaries and assistants just wanted to make them happy. A lot of times people stumble upon an old box of signed photographs in grandma's attic and don't know they're forged."

Barrett, whose specialty is late Hollywood actress Marilyn Monroe's autographs, said that official documents such as contracts and checks are reliable sources to verify whether or not a signature is forged.

"A good rule of thumb is to compare it a signed contract," she said. "Sometimes (celebrities) would have secretaries or other sign photos and letters but they couldn't have a contract signed by a proxy."