Dish Network Corp. (DISH) has increased its bid for mobile-broadband operator Clearwire Corp. (CLWR), topping the offer Sprint Nextel Corp. (S) raised last week.

Late Wednesday, Dish offered to acquire Clearwire for $4.40 a share in cash, a 29% premium over Sprint's offer of $3.40 a share. Clearwire shares jumped 19% to $4.15 after hours.

"The Clearwire spectrum portfolio has always been a key component to implementing our wireless plans of delivering a superior product and service offering to customers," said Dish Chairman Charlie Ergen.

The Wall Street Journal earlier Wednesday reported Dish was preparing to raise its bid for Clearwire.

Clearwire shareholders are to vote on Sprint's takeover offer a share on Friday, but many of them have opposed the terms. Dish said it intends to commence a tender offer for the outstanding Clearwire shares prior to Friday's meeting.

Dish's bid for Clearwire is complicated by the fact Sprint already owns about half the company. Clearwire pointed to that structure Wednesday in a statement rebutting a proxy advisory firm's recommendation that shareholders vote against the Sprint deal.

"Clearwire cannot be sold to another party without Sprint's approval," Clearwire said. "It has been well documented that Sprint is not a willing seller."

The timing of Dish's move is a surprise. The satellite operator had made a separate $25.5 billion bid to acquire Sprint and was just cleared to do due diligence on May 20. Dish appeared to have put its earlier interest in Clearwire on hold while it pursued the bigger deal. Clearwire said in a regulatory filing a week ago that it hadn't had "substantive" talks with Dish since last month.

That said, Mr. Ergen has long put emphasis on Clearwire and its large holding of spectrum--the rights to use the airwaves that all wireless operators need to offer service.

In December, Sprint said it would buy the roughly half of Clearwire that it doesn't own for $2.2 billion, or $2.97 a share. Dish had put in a competing bid the next month at $3.30 a share. Since then, Clearwire's shares had been trading above the initial Sprint price.

A group of Clearwire shareholders, including Mount Kellett Capital Management, Highside Capital Management, Glenview Capital Management and Chesapeake Partners Management, rejected Sprint's latest takeover offer as too low.

Clearwire's board has continued to support the Sprint agreement despite a proxy campaign by Crest Financial arguing against the deal. Despite its holdings, Sprint needs a majority of other Clearwire shareholders to back the deal.

Sprint has said that it is confident in closing the Clearwire deal by July 1.

--Anton Troianovski and Nathalie Tadena contributed to this article.

Write to Sharon Terlep at sharon.terlep@wsj.com and Shalini Ramachandran at shalini.ramachandran@dowjones.com

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