Oct 25 (Reuters) - Sprint Nextel Corp posted a wider third-quarter loss on Thursday as it spent heavily on a network upgrade and lost customers from its iDen network, which it is shutting down.

Sprint posted a loss of $767 million, or 26 cents per share, compared with a loss of $301 million, or 10 cents, in the third quarter a year ago. Sprint has agreed to sell a controlling stake to Japan's Softbank Corp for $20 billion.