Nicole Richie says being spanked by Britney Spears was the ''greatest moment'' of her life.

The 32-year-old star was brought on stage during a performance of Britney's Las Vegas residency in February and she was thrilled to be involved in the stage show because she is such a big fan.

Speaking to Jimmy Fallon on 'The Tonight Show', she said: ''I finally went to go see her in concert. I'm grooving, I'm having the best time.

''All of a sudden, her big security guard just taps me on the shoulder. He says: 'Come with me'. He pulls me up on stage ... and her dancers come over to me and whisper 'Get on all fours' ... It's just something you don't question.''

Nicole - who has children Harlow, six, and Sparrow, four, with husband Joel Madden - then had a harness strapped onto her and was walked like a dog by the 'Toxic' singer, who then pulled out a whip and spanked her.

The 'Simple Life' star admitted she was a massive fan of Britney growing up and regresses back to her teenage years whenever she hears her music.

Nicole said: ''Britney Spears came out when I was 15. And this was the last time in my life that I was like a die-hard fan.

''That love never went away; when I see Britney Spears, I turn into my 15-year-old self.''