Robbie Williams has signed a fan's butt.

The 'Candy' hitmaker pulled luck lady Lone Martinsen up on stage during a concert in Gothenburg, Sweden, up on stage and scribbled his signature across her pert backside after he spotted her saucy poster asking him to put his mark on her.

The 39-year-old singer saw the message, which read, ''Please sign my butt so I can tattoo it,'' while he was performing to a crowd of 65,000 fans on his 'Take The Crown' tour, and told his enthusiastic follower to come up to the stage and bare her bum.

Lone later claimed on Facebook the pop heartthrob told her: ''Your ass is amazing.''

However, the 22 year old admitted she had a restless sleep after the gig, fearing she would smudge the autograph before getting the chance to get it permanently inked on her rear.

She said: ''It is fantastic. But I had a sleepless night though as I dared not shower and it was very hot.

''I've been a big fan for 12 years. To meet him is completely unreal.''

Robbie - who welcomed his first daughter, Theodora, into the world in May 2012 with his wife Ayda Field - is keen to tour more because he ''really loved'' being on the road for his latest gigs.

An insider said previously: ''Robbie didn't want to commit to too many dates at first as he's struggled with touring in the past.

''But he has really loved it this time around and doesn't want the whole thing to end.''