Katy Perry refused to kiss Miley Cyrus because she doesn't know where her ''infamous'' tongue ''has been''.

The 29-year-old singer locked lips with the 'Adore' hitmaker during her Bangerz World Tour last month, but Katy pulled away after Miley attempted to slip her tongue inside her mouth.

Speaking to Australia's 'Sunrise', Katy explained: ''I just walked up to her to give her like a friendly girly kiss, you know, as girls do, and then she like tried to move her head and go deeper and I pulled away.

''God knows where that tongue has been. We don't know! That tongue is so infamous!''

Meanwhile, the 'Dark Horse' hitmaker kicks off her own Prismatic world tour on May 7, so she has put herself on a strict diet in a bid to be in the best shape she possibly can for the shows.

She said: ''It's a lot of work, I have to be very focused. Right now I've cut out all alcohol, all fatty foods, a lot of dairy. I go home and I'm going to have like mostly vegetarian, with some protein on the side like chicken.

''I'm almost 30 guys, my body does not snap back any more!''