Justin Bieber has taken revenge the best way he knows how – by writing a song.

As he hits London to promote his new album Believe, the star reveals the claims that he fathered a child got his creative juices going and inspired him to write a song.

"I'm going to be talking about everything I'm going through. I've written a song about that whole situation with that girl who said she's having my baby - Mariah Yeater.." he told Heat.

Bieber offered to take a paternity test to dispel rumours, at which point Yeater dropped her claims.

At a playback of his new album in London, the star also revealed he wrote a song to thank his fans for their support.

The young heartthrob spent his 18th birthday writing the lyrics, his Beliebers – over 20 million of whom follow him on Twitter – will be happy.

"This is a song I wrote for my fans and it's about how they inspired me," Bieber told The Mirror."It means a lot to me as I wrote it at 12pm on my birthday."

The album is out in June.

Watch the video below to find out what else Bieber did on his 18th birthday...