Google has gone on the offensive against a group of critics that has played a central role in stirring up controversy around alleged fraud in its advertising system.

The company yesterday accused them of relying on flawed technology.

The attack was directed in particular against three of the auditing firms that sell services to advertisers to help them analyse the "clicks" on their keyword advertising on Google and other search engines.

Those audits have become the basis of some of the most notable accusations of click fraud that have been levelled against the search engines.

In a report based on its own investigation of the audit firms' work, Google said it had uncovered "troubling and unexpected" results.

"We discovered some basic engineering and accounting issues across the industry [which have led to] dramatic overestimation of click fraud rates by these firms," the search engine company said.

The attack marks the latest step in a counter-offensive by Google to try to reverse a growing perception that the click-based advertising system, which has been the source of both its own remarkable success, as well as the revival in the fortunes of the entire online advertising industry, is inherently vulnerable to fraud and other failings.

Late last month, the company introduced greater transparency into its AdWords keyword advertising system by promising to give advertisers more information about the number of clicks that Google has judged to be invalid and not charged for. - (Financial Times service)