At least one major Clearwire (NASDAQ: CLWR) investor is not happy about the possibility of Sprint Nextel (NYSE:S) taking advantage of Clearwire's low share price by buying the whole company outright.

Mount Kellett Capital Management, which holds 7.3% of the network operator's voting shares not controlled by Sprint, sent a letter to the Clearwire board -- and also made the letter public in a press release -- expressing that it believes "Clearwire's stock to be substantially undervalued."

Clearwire has been in the process of building out a 4G LTE wireless network, an expensive undertaking. The Mount Kellett letter points out that Clearwire has only enough cash on hand for another year of network construction, and that another $1 billion is needed.

For the Clearwire board to allow Sprint to take over the company before the LTE network build-out is completed would be a dereliction of duty, Mount Kellett said: "In our view, the Board -- each of the members of which owes his or her fiduciary duties to ALL of the stockholders, not just the stockholder that nominated him or her -- has an obligation to take all steps to insure that the Company has adequate cash resources to complete its build-out program and not to allow the Company to reach a point where the only alternative presented is Sprint's acquisition of Clearwire at a price that reflects the Company's unnecessary distress."

For Mount Kellett, the way to raise the needed money for the build-out "is obvious: sell excess spectrum. ... [W]e estimate the Company could generate gross proceeds of $6-$9 billion if it sold all of its excess spectrum."

Not long ago, for Sprint to even consider buying out Clearwire completely would have begged financial credulity. But last month, Sprint accepted the Japanese mobile operator SoftBank's $20 billion offer for a 70% share. That gave Sprint the wherewithal to file with the SEC its intent to increase its 48% slice of Clearwire's voting shares to 50.4%.

The letter also pointed out that Sprint's CEO, Dan Hesse, has said, "Any time there's an opportunity at the right price to take out a strategic investor, we will." According to The Wall Street Journal, Hesse said his company could try to buy up more Clearwire shares.

Mount Kellett, which was founded in 2008 by former Goldman Sachs (NYSE: GS) executives Mark McGoldrick and Jason Maynard, bought its Clearwire shares a year ago. The investment company filed a "Statement of Beneficial Ownership" with the SEC stating it had sent the letter to Clearwire's board.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.