The exodus of senior executives from BlackBerry (Nasdaq: BBRY) continues. 

Mark Cameron, director of global public policy, is leaving to join public- relations firm Hill & Knowlton, marking the latest executive departure for the struggling smartphone maker.

Rick Costanzo, BlackBerry’s chief of global sales, and Chris Wormald, its head of mergers and acquisitions, also are leaving the company, said Lisette Kwong, a BlackBerry spokeswoman. 

Cameron will become a deputy leader of Hill & Knowlton Strategies Canada’s national energy practice in January, the firm said today in a statement. Cameron, a former aide to Canadian Prime Minister Stephen Harper, began working for Waterloo, Ontario-based BlackBerry in early 2011.

The move follows the departure last month of Chief Financial Officer Brian Bidulka, Chief Marketing Officer Frank Boulben and Chief Operating Officer Kristian Tear, part of a shake-up under John Chen, who was named to the top job at BlackBerry on Nov. 4. Chen, Sybase Inc.’s former chief executive officer, joined BlackBerry after a failed attempt by its largest investor to sell the company for $4.7 billion.

Chen is preparing to update investors on his plans to revive the unprofitable company, which has been losing market share to Apple and Samsung Electronics for years.

On Thursday, the company struck a deal to extend a deal with Fairfax Financial Holdings Limited and “the holders of a requisite majority”of the $1 billion of convertible debentures issued by BlackBerry on Nov. 13 that extends until Jan. 13, 2014, the investor option to purchase another $250 million.

BlackBerry operates two research and development offices in Cary.