Apple has warned iOS developers not to use services that manipulate App Store rankings to make their iPhone and iPad apps seem more popular than they really are.

Apple's concern is that app developers are paying companies to use bots to artificially boost apps into Apple's App Store Top 25 charts.

Apple on its Developer page this week issued this warning:

"Once you build a great app, you want everyone to know about it. However, when you promote your app, you should avoid using services that advertise or guarantee top placement in App Store charts.

"Even if you are not personally engaged in manipulating App Store chart rankings or user reviews, employing services that do so on your behalf may result in the loss of your Apple Developer Program membership. Get helpful tips and resources on marketing your apps the right way from the App Store Resource Center."

Apple says its App Store has more than 500,000 apps and that customers have downloaded more than 18 billion apps.