Katy Perry has congratulated boyband One Direction on their incredible success since she appeared a guest judge on The X Factor. 

The pop princess was partly responsible for putting the 'What Makes You Beautiful' singer Niall Horan through to the next round on the show during her short stint on the panel, and the lads' debut album, 'Up All Night' has now soared straight to No.1 on the US iTunes chart following its release. 

The 'Teenage Dream' songstress tweeted a message to Horan that read: "Congratulations, you didn't let me down! xo"

Soon after, he replied: "Hey katy, thank you,let's hope it holds out this week! Thank you for putting me thru! We've been working hard, I won't let you down."