NEW YORK (Reuters) - Clearwire Corp said its 2012 revenue may fall, or at best remain at 2011 levels, as it faces increased competition at its biggest customer Sprint Nextel, sending its shares down more than 5 percent in late trade.

The wireless service provider, which is majority-owned by Sprint, also forecast higher capital spending this year as it kicks off an upgrade to its network.

Clearwire forecast 2012 revenue of $1.15 billion to $1.25 billion compared with its 2011 revenue of $1.25 billion.

It also forecast an adjusted loss before interest, tax, depreciation and amortization of $250 million to $350 million and said capital spending would $450 million to $550 million, with most spending coming in the second half of the year.

For the fourth quarter, Clearwire posted a wider quarterly net loss as costs increased and it wrote down the value of certain assets including a charge related to a change of its preferred network technology.

Clearwire posted a loss of $236.85 million or 81 cents per share compared with a loss of $128.01 million, or 79 cents per share in the year ago quarter. Revenue rose to $361.9 million from $175.2 million.

Clearwire shares fell to $2.23 in late trade after closing at $2.36 on Nasdaq.

(Reporting By Sinead Carew; Editing by Bernard Orr)