Google is launching a new advertising system that generates cash when browsers buy an item rather than just clicking on an advertisement. 

The system is an extension to its current pay-per-click Adsense programme, which allows website publishers to display adverts from Google and receive revenue when visitors click on the ads. 

The new offering is based on a 'cost-per-action' model. Website owners will only receive money when visitors complete a specific action determined by the advertiser, such as purchasing an item or signing up for a service.

Google is increasingly facing problems with click fraud on its Adsense programme, where a person or automated script clicks on pay-per-click advertisements for financial gain. The search giant recently settled a case for $90m. 

A cost-per-action model is less vulnerable to such fraud, since users need to perform a specific task.

The trial is being offered to selected website owners currently participating in the Adsense programme.

In an email sent to David Jackson of financial analysts Seeking Alpha, Google explained that the ads are "very different in that [website owners] will be able to choose amongst a selection and have more flexibility in promoting them". 

The email went on to explain that the new system would run separately to Google's current offerings.

"These ads will not compete with contextually targeted ads. Instead, they will show across a separate Content Referral network," said Google.

The move puts Google in direct competition with established player ValueClick which owns affiliate network Commission Junction.