Yahoo! Search Marketing has begun notifying U.S. advertisers that they will no longer be allowed to bid on competitors' trademarked keywords beginning this week.

In the past, advertisers were allowed to bid on competitors' keywords if their site provided detailed information comparing the trademarked item to its own, in order to help consumers make a decision. Yahoo! this week began notifying advertisers that had previously bid on trademarked keywords, explaining in an e-mail that the policy would change on March 1.

"In order to more easily deliver quality user experiences when users search on terms that are trademarks, Yahoo! Search Marketing has determined that we will no longer allow bidding on keywords containing competitor trademarks," the message states.

A likely reason behind the move is Yahoo!'s desire to court more brand advertisers to link offline campaigns to search, while protecting the trademark owner from competitors who "hijack" the keyword with their own ads, Search Engine Watch's Danny Sullivan wrote in the site's blog.

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