(Reuters) - Sprint Nextel Corp on Monday said it has sued Dish Network Corp to block its tender offer for Clearwire Corp, escalating a takeover battle that also includes Japanese mobile carrier SoftBank Corp.

Sprint, the third-largest U.S. phone company, in a statement said Dish's offer violated Delaware law, and was part of an effort to "fool" Clearwire shareholders and block Sprint's competing effort to obtain Clearwire spectrum. It said it sued in Delaware Chancery Court to block the tender offer.

The lawsuit comes amid a battle in which SoftBank and Dish are bidding for Sprint, while Dish and Sprint are bidding for Clearwire, a wireless broadband provider.

Sprint also named Clearwire in the complaint.

Last week, SoftBank raised its offer for Sprint to $21.6 billion from $20.1 billion, which would give it a 78 percent stake in the company.

Sprint then gave Dish, which is controlled by billionaire Charlie Ergen, until June 18 to sweeten its earlier $25.5 billion bid, which it said is not "actionable," and make its best and final offer.

Dish did not immediately respond to requests for comment about Sprint's lawsuit, a copy of which was not immediately available.

Clearwire spokeswoman Susan Johnston said that company does not discuss pending litigation.

Last week, Clearwire's board urged shareholders to accept the Dish tender rather than an earlier buyout by Sprint, its majority owner.

Shareholders of Sprint are scheduled to vote on the latest SoftBank offer on June 25.

Paulson & Co, the hedge fund firm run by billionaire John Paulson and Sprint's second-largest shareholder, has said it would vote for the SoftBank transaction

(Reporting by Jonathan Stempel in New York and Sruthi Ramakrishnan in Bangalore; Editing by Andre Grenon and Stephen Coates)

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