Clearwire Corp., the company building out a high-speed WiMax wireless network, fell in Nasdaq Stock Market trading after disclosing that it may not have enough funding to keep operating its business.

“Our expected continued losses from operations and the uncertainty about our ability to obtain sufficient additional capital raises substantial doubt about our ability to continue as a going concern,” Clearwire said.

Clearwire is cutting 15 percent of its workforce, reducing sales and marketing spending and delaying its Clear-branded smartphone as part of a plan to save between $100 million and $200 million this year. The Kirkland, Wash.-based company has 4,200 employees, putting the cuts at about 630 jobs.

Clearwire fell 24 cents, or 3.3 percent, to $6.93 yesterday. The stock has risen 2.5 percent this year. Sprint Nextel Corp., which owns 54 percent of the company, lost 10 cents, or 2.4 percent, to $3.99. The shares have gained 9 percent this year.

Clearwire said it may run out of cash as early as mid-2011. The plan should save $100 million to $200 million more in the first half, the unprofitable company said. That still won’t be enough to cover the shortfall, according to the filing.

Sprint may be forced to fund Clearwire to keep it from defaulting, said Craig Moffett, an analyst at Sanford C. Bernstein & Co. in New York. He said Sprint may need to invest $1 billion to $2 billion in Clearwire in the next three to four years.

Clearwire got $1.56 billion from Sprint and other investors last year to help it expand its high-speed mobile Internet network.