Donald Trump has warned Katy Perry against John Mayer.

The billionaire businessman has bizarrely taken to offering relationship advice, telling the 'I Kissed a Girl' singer she should be wary of John, with whom she has had an on/off relationship since the summer.

He tweeted: ''@katyperry is no bargain but I don't like John Mayer. he dates and tells - be careful Katy (just watch!) (sic).''

Donald does have a point, as John has dated a number of famous women, including Jessica Simpson, Jennifer Aniston, Cameron Diaz and Jennifer Love Hewitt, and not always been private about it. He once famously described having sex with Jessica as ''like crack cocaine to me.''

However, he was upset when made the subject of the track 'Dear John' by another of his famous ex-girlfriends, Taylor Swift.

He said: ''I was really caught off-guard, and it really humiliated me at a time when I'd already been dressed down. I mean, how would you feel if, at the lowest you've ever been, someone kicked you even lower?

''It made me feel terrible. Because I didn't deserve it. I'm pretty good at taking accountability now, and I never did anything to deserve that. It was a really lousy thing for her to do.''

John also later apologised for his indiscretions, saying: ''I did a couple of really dumb interviews. It kinda woke me up. It sort of rocketed me into adulthood.''

Katy and John got together in July, her first proper relationship since splitting from husband Russell Brand at the end of last year.