"We used to have a click-through rate of 1 to 2 per cent for each time one of our ads was shown," said Oscar Jenkins, chief executive of Dynmark, a company that sells bulk text messaging services.
"Then suddenly in October it increased to around 60 to 70 per cent. We had exceeded our advertising budget by 10 in the morning."
Google has advised the companies to increase their advertising spend to avoid burning through their budgets so quickly.
But for some of the smaller companies, such as Dynmark, this is not an option. Instead Dynmark has temporarily halted much of its advertising campaign.
The problem is not yet very widespread in the UK. Dave Harrison, founder and president of WSPS, one of the country's largest buyers of search advertising, says his company monitors millions of clicks each day across a number of sectors, and has only detected one serious click-fraud problem in the last three months.