Screenshot by Steven Musil/CNET

Microsoft is ratcheting up its attacks on Google with a campaign urging users of the Web giant's e-mail service to dump Gmail for its own Outlook.com over privacy concerns.

In its national campaign titled "Don't get scroogled by Gmail," Microsoft dredges up an old issue with Google's free e-mail service: Google scans users' e-mails to determine relevant advertisements to place alongside the messages.

Microsoft says a study it commissioned found that 70 percent of consumers polled were unaware that major e-mail providers "routinely" scan e-mail to sell ads and that nearly 90 percent disapproved of the practice, which has been criticized in the past.

"Outlook.com believes your privacy is not for sale," Stefan Weitz, senior director of Online Services at Microsoft said in a statement. "We believe people should have choice and control over their private email messages, whether they are sharing banking information or pictures of their family or discussing their medical history."

The anti-Gmail effort is the latest salvo at Google. The European Commission has spent the past two years investigating Google after competitors such as Microsoft complained that Google adjusted search results to bring up its products and Web sites first. The U.S. Federal Trade Commission recently found that Google's search results were not biased in favor of its own results in a way that was anticompetitive -- a decision that was largely seen as a victory for the Web giant.

The Scroogle campaign is apparently ongoing. In late November, Microsoft launched Scroogled.com and took aim at Google Shopping search results, which are paid for by companies.

Gmail, introduced in 2004, was immediately slammed as a horrific invasion into Internet users' privacy by lawmakers and privacy advocates alike. The critics contended that it should be illegal for a company to scan the text of its customers' e-mail correspondence and display relevant advertising. The practice has led to occasional lawsuits.

However, Google has long maintained that its automated scanning technology does not invade users' privacy.

"Advertising keeps Google and many of the websites and services Google offers free of charge," Google said in a statement. "We work hard to make sure that ads are safe, unobtrusive and relevant. No humans read your email or Google account information in order to show you advertisements or related information."

As of October, Gmail was a close-second to Yahoo as the No. 1 free e-mail provider in the U.S., claiming 69.1 million users to Yahoo's 76.7 million, according to ComScore. Microsoft's Hotmail was a distant third with 35.5 million, according to the market researcher. Microsoft announced last year that Hotmail would be phased out for a rebranded Outlook.com.

While Microsoft also shows ads with its Outlook service, the tech giant asserts it automatically scans the contents of users' e-mails only to prevent spam, malware, and other unwanted activity.