(Reuters) - Sprint Nextel must give AT&T hundreds of thousands of additional documents that the giant telecommunications company wants for a legal fight with the Department of Justice over whether it can buy smaller competitor T-Mobile USA, a special master in the case has ruled.

Richard Levie, a special master overseeing document production in the Justice Department's challenge of the proposed $39 billion merger, on Sunday rejected Sprint's attempt to quash AT&T's subpoena of the documents.

Instead, Levie ruled that Sprint had to update documents that it had previously given to the Justice Department, which were subsequently given to AT&T, as well as provide additional papers.

The special master noted, in particular, that Sprint had acquired rights to sell Apple's iPhone since the Justice Department filed its lawsuit to stop AT&T's acquisition of T-Mobile.

"AT&T is entitled to discover what effect the iPhone and other events of the past few months have had on Sprint's relevant market share, a part of the government's .... case," Levie wrote in his order.

In particular, AT&T asked for documents describing Sprint's assessment of T-Mobile's competitive significance, plans to target T-Mobile customers and negotiations with device makers about exclusive or preferential rights.

Sprint, which has separately sued AT&T to stop the deal, had objected to the document production.

The deadline for document delivery is November 21, and the case goes to trial on February 13. Levie estimated that Sprint would give AT&T about 440,000 pages of documents.

Sprint has argued that the document production would also be helpful to AT&T in defending itself against Sprint's lawsuit. Sprint has not yet begun discovery in that case.

The Justice Department in August sued to block the deal, which would vault No. 2 ranked AT&T into the leading position in the U.S. wireless market. T-Mobile is the No. 4 operator.

Sprint, the No. 3 U.S. carrier, has argued that AT&T's purchase of T-Mobile USA would create an overwhelming duopoly that would make it impossible for Sprint to compete effectively.

The current industry leader is Verizon Wireless, a venture of Verizon Communications Inc and Vodafone Group Plc.

A key government concern is that T-Mobile generally costs less than other carriers so its disappearance could mean higher prices for wireless service.

The cases are USA v. AT&T, T-Mobile USA Inc and Deutsche Telekom AG, case No. 11-1560 and Sprint Nextel Corp v. AT&T Inc et al, No. 11-1600, and Cellular South v. AT&T, No. 11-1690. Both are before the U.S. District Court for the District of Columbia.

(Reporting by Diane Bartz, editing by Maureen Bavdek)