Security specialist F-Secure is warning all users of iTunes to be careful when devising passwords for their Apple Store accounts.

The advice comes after iPhone developer Thuat Nguyen was caught using compromised iTunes accounts to artificially inflate sales of his applications. Apple has since banned the developer from the iTunes Store.

"Now some might think using the same password for email and iTunes isn't a problem, because the store is linked to your iPod, right?" F-Secure notes in a blog post.

"But there is a problem, the store is also linked to your credit card. The Nguyen case clearly demonstrates, if there's a will there's a way, to steal from your credit card."

According to F-Secure, around 20 per cent use the same password for various accounts. The survey covered Internet users in the UK, Germany and Sweden.

F-Secure offers antivirus software, Anti Spyware, firewall and Internet security tools for home users and businesses.