Smartphones will become the number one target for cyber criminals within 5 years, according to security experts.

Hackers are developing and using viruses such as 'Trojan horses' to gain undetected access to a phone's internal systems, where they can see every key stroke entered by a user.

This has serious implications for those who use their phones to access bank accounts and apps that hold sensitive, personal information. Roughly a quarter of the world's population own smartphones.

Keiron Shepherd is a senior security engineer at the world leading cyber security company F5 Networks. He has been monitoring the targeting of mobile devices by hackers.

He told Sky News: "If you just consider the amount of smartphones and the number of people, it's a great surface area for attackers to go for.

"Windows was the predominate system, it was the path of least resistance for the malware writers. Devices and operating systems which were considered not an issue to be worried about in the past have now become a target for the malware writers."

One virus monitored by F5 Networks imbeds in an innocent-looking advert on a website. When clicked, the virus infiltrates their device and monitors every key stroke, even when the user accesses their bank accounts.

Keiron Shepherd said: "The way this virus can insert itself between the applications you're using before it accesses the internet gives it a chance to extract critical data such as credit card numbers, bank accounts; anything that's of high value.

"It really is a numbers game. They'll throw enough malware out there and hope it returns a good investment."