Yahoo general counsel Michael Callahan is stepping down “to move on to new opportunities,” according to a resignation letter filed with the Securities and Exchange Commission, reports The Recorder.

Ron Bell, deputy GC for products and the Americas region, is serving as interim general counsel, according to an internal memo from CEO Ross Levinsohn.

“In my time with Yahoo, Mike has been a leader and great partner to me and the entire executive team and we will miss his calm demeanour and practical, results-oriented advice,” Levinsohn wrote. “Along with the board, I would like to thank Mike for his loyalty and service, and wish him well in whatever comes next for him.”

Callahan has been the search engine’s top lawyer since 2003. He was in charge of worldwide legal affairs and public policy and advised Yahoo’s top management and board on strategic and corporate governance.

Sources say that Callahan, under former CEO Scott Thompson’s direction, was a driving force in Yahoo’s decision to sue Facebook over online advertising patents, a suit the companies are in the process of settling, according to court filings.

Callahan is the latest in a string of departures from the company’s legal department. Yahoo’s California-based legal team has shrunk by nearly 20% since January 2011. Former deputy GC Belinda Johnson had been with Yahoo for more than 12 years when she joined online accommodations site Airbnb as its GC in December.

Two in-house lawyers left for rival Facebook: Yahoo’s former senior legal director of privacy, Joshua Smith, joined Facebook as a lead privacy counsel in February and former senior legal director Susan Cooper spent four years at Yahoo before joining Facebook in April 2011 as lead advertising and product counsel.

Callahan joined Yahoo in 1999 after briefly serving as corporate counsel for Foster City’s Electronics for Imaging Inc. Before that, Callahan was an associate for several years at Skadden Arps Slate Meagher & Flom, handling M&A and corporate finance.

Callahan, whose departure is effective as of 9 July, declined to comment through a company spokeswoman.

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