Lady Gaga was a surprise performance at the annual Royal Variety Show at London's Hammersmith Apollo on Tuesday night. Dressed in an elegant Brandon Maxwell gown, Gaga took to the stage to perform a beautiful rendition of her latest hit song "Million Reasons" in front of Prince Charles and The Duchess of Cornwall.

After the show, Gaga lined up with the rest of the performers to be greeted by the royals. As they were introduced, Camilla Parker-Bowles, the Duchess of Cornwall told the Joanne singer: "My grandchildren call me Gaga."

Watch Gaga sing "Million Reasons" below and let us know what you think in the comments...