Katy Perry's collaboration with Rihanna is taking so long because ''you have to wait for greatness.''

The 'Part of Me' singer has planned to record a song with her friend for a long time, but it is taking time because they are searching for the right track.

During a special performance on the UK's Radio 1, Katy said: ''We collaborate on a lot of things, just not songs yet - we have a great Friendship.

''Its' one of those things where it's gotten so much build up that if we don't' deliver, like, the best song... We're both busy, but we planted a seed two years ago, we're just looking for that special one.

''How disappointed would you be if that song came out and it wasn't that good? So sometimes you have to wait for greatness.''

Katy explained she would like the track to be iconic and empowering, rather than sexy, like Shakira and Beyonce's 2007 single 'Beautiful Liar'.

She added: ''I was telling her, you know the Eurythmics song featuring Aretha Franklin, 'Sisters are Doing it for Themselves'? That's like the kind of song which I'd like to do, rather than 'Beautiful Liar' - although that is still a great song.

''I loved that but I definitely want to do something which is iconic.''