Sprint Nextel (S) has begun the process of taking control of its longtime networking partner Clearwire (NASDAQ: CLWR).

The third-largest U.S. mobile carrier made a regulatory filing dated today indicating that it intends to buy enough shares from Eagle River Holdings to give it a 50.4% voting interest in the perennially struggling Clearwire. Sprint currently owns 48% of Clearwire's voting shares.

The impetus to seek control of Clearwire seems to be the agreement by Japanese mobile operator Softbank to buy 70% of Sprint for $20 billion. According to The Wall Street Journal,  people familiar with the matter have said the deal came with a caveat from Softbank's financiers that Sprint would have control of Clearwire and its cache of wireless spectrum licenses.