Lady Gaga and Madonna

They started out as best of friends when Lady Gaga admitted she owed a great deal to Madonna for blazing a trail for female pop stars, but things got catty with the release of Born This Way, with its similarities to Madonna's 1989 smash Express Yourself.

The mutual appreciation society turned sour when Madonna said she was sceptical of Lady Gaga's motivations, saying, "I have no comment to make about her obsessions having to do with me because I don't know whether her behaviour is rooted in something deep and meaningful or superficial." The queen of pop has also used her MDNA tour to mock Gaga on stage.