Troubled pop star Britney Spears has appeared on her favourite U.s. sitcom How I Met Your Mother.

The Toxic hitmaker made a cameo appearance on the show - playing a secretary opposite Neil Patrick Harris - which aired on Monday night.

Spears, 26, is a huge fan of the show, and deemed a guest role in the series a perfect assignment at this juncture of her turbulent career.

And U.s. TV critics have been complimentary of her performance, describing her appearance as "fresh-faced and relaxed".