Google on Tuesday went on the offensive against a group of critics that has played a central role in stirring up controversy around alleged fraud on its advertising system, accusing them of relying on flawed technology.

The attack was directed in particular against three of the auditing firms that help advertisers analyse the “clicks” on their keyword advertising on Google and other search engines. Those audits have become the basis of some of the most prominent accusations of click fraud.

In a report based on an analysis of these 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.

While one of the firms conceded that criticisms were valid and said it had since improved its technology, the others said they were still reviewing Google’s report, and one questioned its accuracy.

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 is vulnerable to fraud. It recently promised to give advertisers more information about the clicks that Google believes are invalid and has not charged for. Last week, it joined with a number of other big online companies to work on a standard measurement system that would determine when search engines charge advertisers for clicks, and how they decide which clicks are fraudulent.

In its report on Tuesday, Google took aim at the auditing practices of ClickFacts, Click Forensics and AdWatcher, three of the independent auditing firms used by advertisers to assess the effectiveness of their keyword advertising. The audits are sometimes used by advertisers to challenge Google’s billing practices and have also formed the basis of public criticisms of its vulnerability to fraud.

One common flaw, according to Google, involved the auditors’ software counting multiple clicks on an advertiser’s site from a single visitor, even though the same activity only produced one click, and therefore one payment, in Google’s records. It also said that “cookies” placed on users’ computers to track their online behaviour had counted clicks on other adverts, or on other search engines, and included these when reporting on an advertiser’s online traffic.

Eugene Mordkovich, one of the founders of AdWatcher, said in an e-mail that while the company was still reviewing Google’s criticisms, “some of the points don’t seem to make sense”. AdWatcher’s technology does not count multiple hits on a single site, he said, and added that it would be wrong to claim that his company mixes up results from different search engines.

Mikhail Ledvich, a ClickFacts executive, said that the flaws highlighted by Google had been based on a “beta”, or test, version of his company’s software in February this year. “The issues they bring up have since been solved,” he said.

A spokesman for Click Forensics said it was still reviewing Google’s report.