The Rolling Stones may be very open on subjects like drugs and promiscuity, but the one question that is off limits is Mick Jagger's penis.

Mick Jagger. Pic/AFP

Keith Richards had mentioned Jagger's small manhood in his autobiography, Life. Since then the band's PR has made sure no mention of it is made again.

"(it was) the only time genitalia size has been listed as a verboten topic in any interview I''ve ever done," quoted Elizabeth Day, a music writer for The Observer Newspaper as saying.

Day, who has written a 3000-word piece on the band, says that Rolling Stones' PR told her that she was not allowed to ask about 'big and small willies'.