Many net firms were criticised in the report for privacy failings
Rights group Privacy International rated the search giant as "hostile" to privacy in a report ranking web firms by how they handle personal data.
The group said Google was leading a "race to the bottom" among net firms many of whom had policies that did little to substantially protect users.
In response Google said the report was mistaken and that it worked hard to keep user data confidential.
The report by the veteran cyber rights group is the result of six months' research which scrutinised 20 popular net firms to find out how they handle the personal information users gave up when they started using such services.
None of the firms featured in the report got a "privacy friendly" rating.
Yahoo and AOL were said to have "substantial threats" to privacy as were Facebook and Hi5 for the allegedly poor way they dealt with user data.
Other net sites, such as BBC.com, eBay and Last.fm were described in the report as "generally privacy aware but in need of improvement".
But Privacy International singled put Google at the bottom of its rankings for what the group called its "numerous deficiencies and hostilities" to privacy.
"We are aware that the decision to place Google at the bottom of the ranking is likely to be controversial," the group said in the report.
Privacy International placed Google at the bottom of its ranking because of the sheer amount of data it gathers about users and their activities; because its privacy policies are incomplete and for its poor record of responding to complaints.
"While a number of companies share some of these negative elements, none comes close to achieving status as an endemic threat to privacy," read the report.
Responding to the report Nicole Wong, general counsel for Google, said in a statement: "We are disappointed with Privacy International's report which is based on numerous inaccuracies and misunderstandings about our services."
Ms Wong added: "We recognise that user trust is central to our business and Google aggressively protects our users' privacy."
Privacy International said it planned to release a more detailed report in September produced after detailed consultation with the firms covered in the first draft.