A new study released on Tuesday shows that many top websites are sharing personal user information to at least four other websites.

At least 45 percent of the top 185 most-visited websites share user information like user name or user ID with another website. The study by Jonathan Meyer, a graduate student in law and computer science in Stanford University concludes that users have no idea that websites are regularly sharing personal information between them.

"Many first-party websites and third parties make what would appear to be incorrect representations about not sharing or collecting 'personally identifiable information,'" Mayer said in a written statement.

The study also shows that websites don't just trade information about user ID or user name but shares more personal information including gender, age, ZIP code and relationship status through referrers. An example in the study is the dating website OKCupid which shared the gender, age and relationship status of its users to two companies that sell online personal information, BlueKai and Lotane.

The report was presented as part of an effort by advocacy groups to pressure the Obama government to regulate internet companies and their use of private information.

The data analyzed was collected when Meyer set up new accounts in the said sites. He found that websites transmitted his information to at least one outside website. Rottentomatoes.com transmitted the most data to outside domains; it sent information to at least 83 other domains.

In the Stanford study, Mr. Mayer said he set up new accounts at each site and surfed each website for about 15 minutes.

He found that 61% of the websites transmitted identifying information to at least one outside web domain. But he did not check whether those outside web domains were owned by the same company (such as YouTube sending data to its parent company Google).

He found that 45% of sites transmitted to at least four separate domains - which he said was a more robust representation of the amount of data sent to outside companies.

The websites that transmitted the most data were Rottentomatoes.com, which sent identifying information to 83 domains and Cafemom.com, which sent to 59 domains.

ComScore's scorecardsearch.com and Google's google-analytics.com were the top two recipients of the information.

ComScore responded by saying that any information they received was strictly used for market research and that the personal information is destroyed after it is compiled.

"We've never attempted or wanted to parse out personal information in any URL schema provided by a third party site," Google told the Wall Street Journal.