The watchdog gave Google the lowest score of all the major web firms in the report, published this weekend.
The bone of contention seems to involve one member of the watchdog's 70-member international advisory board who was head-hunted by Microsoft after joining Privacy International.
"Privacy International is and has always fought hard for its independence, often to our own great expense," Davies told Google in the letter.
"Can I be so bold as to suggest that your company's actions stem from sour grapes that you achieved the lowest ranking amongst the internet giants?
"We have no specific axe to grind with Google. It is one of many companies demonstrating a poor privacy performance, and in assessing that performance we are acting solely with the intention of raising public awareness."
Davies leaves the ball firmly in Google's court, requesting an apology from the web giant.
"I believe an apology from you is in order, but if you cannot deliver this then I think you should reflect carefully on the actions of your representatives before embarking on what I believe amounts to a smear campaign," he said.
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