Google was caught last week bypassing default privacy settings in the Safari browser in order to serve up tracking cookies. The company claimed the situation was an accident and limited only to the Safari web browser, but today Microsoft claimed Google is doing much the same thing with Internet Explorer.

In a blog post titled “Google bypassing user privacy settings” Microsoft’s IE Corporate Vice President Dean Hachamovitch states that “When the IE team heard that Google had bypassed user privacy settings on Safari, we asked ourselves a simple question: is Google circumventing the privacy preferences of Internet Explorer users too? We’ve discovered the answer is yes: Google is employing similar methods to get around the default privacy protections in IE and track IE users with cookies.”

Hachamovitch explains that IE’s default configuration blocks third-party cookies unless presented with a “P3P (Platform for Privacy Preferences Project) Compact Policy Statement” indicating that the site will not use the cookie to track the user. Microsoft accuses Google of sending a string of text that tricks the browser into thinking the cookie won’t be used for tracking. “By sending this text, Google bypasses the cookie protection and enables its third-party cookies to be allowed rather than blocked,” Microsoft said.

The text allegedly sent by Google actually reads “This is not a P3P policy” and includes a link to a Google page which says cookies used to secure and authenticate Google users are needed to store user preferences, and that the P3P protocol “was not designed with situations like these in mind.”

Microsoft said it has contacted Google to ask the company to “commit to honoring P3P privacy settings for users of all browsers.” Microsoft also updated the Tracking Protection Lists in IE9 to prevent the tracking described by Hachamovitch in the blog post. Ars has contacted Google to see if the company has any response to the Microsoft allegations, and we’ll update this post if we hear back.

UPDATE: It turns out Facebook and many other sites are using an almost identical scheme to override Internet Explorer’s privacy setting, according to privacy researcher Lorrie Faith Cranor at Carnegie Mellon University. “Companies have discovered that they can lie in their [P3P policies] and nobody bothers to do anything about it,” Cranor wrote in a recent blog post.

UPDATE 2: Google has gotten back to us with a lengthy reply, arguing that Microsoft’s reliance on P3P forces outdated practices onto modern websites, and points to a study conducted in 2010 (the Carnegie Mellon research from Cranor and her colleagues) that studied 33,000 sites and found about a third of them were circumventing P3P in Internet Explorer.

“Microsoft uses a ‘self-declaration’ protocol (known as ‘P3P’) dating from 2002 under which Microsoft asks websites to represent their privacy practices in machine-readable form,” Google Senior VP of Communications and Policy Rachel Whetstone says in a statement e-mailed to Ars. “It is well known—including by Microsoft—that it is impractical to comply with Microsoft’s request while providing modern web functionality.”

Facebook’s “Like” button, the ability to sign into websites using your Google account “and hundreds more modern web services” would be broken by Microsoft’s P3P policy, Google says. “It is well known that it is impractical to comply with Microsoft’s request while providing this web functionality,” Whetstone said. “Today the Microsoft policy is widely non-operational.”

That 2010 research even calls out Microsoft’s own msn.com and live.com for providing invalid P3P policy statements. The research paper further states that “Microsoft’s support website recommends the use of invalid CPs as a work-around for a problem in IE.”

This article originally appeared on Ars Technica, Wired’s sister site for in-depth technology news.