What started as tensions between a privacy campaign group and Google over its controversial mapping service Street View has spilled over into claims of conspiracy theories.

Simon Davies, director of Privacy International, believes Google is trying to discredit the complaints he has registered about Street View by briefing against him to journalists.

In an open letter to Google's chief executive Eric Schmidt, Mr Davies accused it of secretively briefing against him, claiming he was supported by, and biased in favour of, Microsoft – one of Google's biggest competitors. Google has openly affirmed its belief in the relevance of connections between Microsoft and 80/20 Thinking, a data protection consultancy run by Mr Davies.

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"Neither Microsoft nor [its PR company] Burson-Marsteller has ever paid money to either Privacy International or 80/20 Thinking, nor has any benefit in kind been given," Mr Davies wrote in his letter. "We are quite frankly stunned that a company such as Google would take steps... to peddle groundless conspiracy theories in an attempt to besmirch a critic. You should be ashamed of your actions. Google is coming across as a desperate company resorting to desperate measures."

In response, Google has persisted with the assertion that Mr Davies' connections to Microsoft should be made clear in public. A spokesman said: "Simon Davies regularly attacks Google on privacy grounds. It's no secret that we believe the credibility of his criticisms is undermined by the fact that, alongside his work for Privacy International, he acts as a consultant to a number of technology companies who are direct rivals to, and in some cases vocal critics of, Google – a fact he rarely seems to disclose in his press releases or comments to the media. We work hard to make sure our users understand what data we collect and how we use it, because we are committed to transparency and user choice."

80/20 Thinking's advisory group includes Jerry Fishenden, Microsoft UK's leading technology adviser, but has never included anyone from Google.

Privacy International has previously protested against other Google technology, such as Latitude, which allows users to see their friends' locations, and its advertising outfit Doubleclick. The campaign group submitted an in-depth letter of concern to the Information Commissioner's Office on Monday in relation to Street View.

The service, which was introduced in the UK last week, shows 360 degree images of streets from 25 British cities. Faces and car numberplates are supposed to be blurred out but in some cases can be seen. Users can complain to Google if they find images on Street View offensive. The images can usually be taken down within hours.

Mr Davies said Privacy International had received several hundred complaints about the service, mostly in relation to the failure of the blurring-out software. "I don't want Google punished and I don't want Street View removed," he said. "We'd be happy if Google just made a commitment to be more transparent and accountable. The current situation is not sustainable."

Microsoft declined to comment.