Advertisers pay each time someone clicks on their advert, but fraudsters can push up competitors' costs by deliberately clicking on their ads.

Some organisations estimate click fraud at over 30% of all traffic, as reported by Techworld.

Google has set up a website so that its advertisers can learn about the issues.

According to Techworld, the search engine giant has researched the extent of the problem on its own sites and concludes that less than 10% of clicks on Google adverts are invalid.