The bronze plaque inscribed with Mercury's real name, Farrokh Bulsara, and signed by "M", believed to stand for former girlfriend Mary Austin, went missing from a plinth at the Kensal Green Cemetery.
It is believed Austin may have taken down the memorial after its existence was made public last month (Feb13).
A source tells Britain's Daily Mirror, "It's just disappeared into thin air. No sooner had the mystery of Freddie's ashes seemingly been solved than this adds a whole new chapter. It's all very odd but just adds to the intrigue."
The rocker died of an AIDS-related illness in 1991 when he was just 45, and fans have spent years trying to ascertain where exactly his ashes were scattered, with some reports suggesting they were taken to Zanzibar, the country of his birth, or Lake Geneva in Switzerland, where he owned an apartment.