Billionaire Carl Icahn has upped the pressure on struggling Motorola, revealing today in a proxy filing with the SEC that he has increased his stake in the handset and network vendor and thus adding more legitimacy to his claims at board seat.

Icahn and his affiliates now own 2.7% of Motorola’s shares up from 2.48% reported earlier this month. Though that hardly approaches a controlling interest, for every share Icahn and his consortium buys, he receives one more vote for himself as a board member and puts further pressure on other shareholders to back Icahn’s coup. Icahn has if he wins the seat he will use his vote and influence to force Motorola to buy back $12 billion to $15 billion in shares, which Icahn maintains are cheap due to Motorola’s recent poor performance.

The filing came just two days after Motorola took another hit in the market after projecting a first quarter loss in profits and a shake-up in upper management. Motorola chief financial officer David Devonshire is retiring in the middle of what may be a dismal financial cycle for the vendor, and networks and enterprise head Greg Brown was tapped as president and chief operating officer, replacing Ron Garriques who left in February to work at Dell. At the same earnings warning call, Motorola also announced an accelerated $2 billion share repurchasing plan with the goal of buying back $7.5 billion in stock by July of 2009, essentially meeting Icahn’s demands halfway.

After almost two-years of rapid growth driven primarily by sales of the RAZR, Motorola’s fortunes took sharp turn last year after it began projecting slower growth in its primary handset business. Its networks business had already been suffering as it has had difficulty competing in the 3G business against its larger and rapidly consolidating CDMA and UMTS counterparts. Just as Motorola found a new direction in networks in WiMAX—landing contracts with Sprint, Clearwire and others—it began to lose direction in handsets, despite over a year or trying to find the successor to the RAZR. Despite Motorola’ significant gains in the global handset market, Nokia still managed to keep its market share in the sector. Now that Motorola’s momentum has slowed, Nokia has hinted it will push back even harder on the U.S. vendor.