Billionaire investor Carl Icahn has increased his stake in Motorola. According to a filing with the Securities and Exchange Commission, Icahn now holds 6.3% of Motorola’s shares, up from 1.4% last March. Last month, Icahn’s stake was 5%.

Last year, Icahn waged a proxy battle trying to secure a seat on the struggling handset maker’s board. He also publicly called for former CEO Ed Zander to step down. More recently, Icahn nominated four directors to stand for election at Motorola’s 2008 annual meeting of stockholders. Although the company has not yet scheduled its yearly meeting, Motorola has urged shareholders to reject Icahn’s nominees.

Icahn also has suggested that the company should sell off its handset business, while Motorola’s new CEO Greg Brown said the company is committed to a “product-led” recovery.