Hackers are making money by upping their attacks on online pay-per-click advertising services like those run by Google and Yahoo. 

The rise of the cyber-thieves has been blamed on an increase in the use of bots that infect people's computers and make it look like hundreds of users are clicking on adverts when they are not.

"We are seeing that, as the pay-per-click industry gets more sophisticated and the fraudsters get more sophisticated, there are an increasing number of botnets," Shuman Ghosemajumder, product manager for trust and safety at Google, told the Wall Street Journal.

The problem is a concern for companies selling ads on a per-click basis because they must convince advertisers that the audience is real.

However, the small scale of the problem, estimated to be less than five per cent of click-throughs at the moment, and the ability to change the bot programs quickly, makes it hard for the fraud to be discovered.

Advertising companies, however, believe that they can fight against an area that is expected to grow.

"We have always seen this as a challenge, but we have always seen this as a manageable one," Yahoo spokeswoman Gaude Paez told the Wall Street Journal.

Yahoo and Google said they both catch the majority of false clicks and that advertisers are refunded for any bad clicks that were missed originally.