Shares of Staples Inc. (Nasdaq: SPLS) and Office Depot Inc. (Nasdaq: ODP) tumbled in early trading after a federal judge blocked the combination of the two largest office suppliers, saying it would create an unrivaled giant.

U.S. District Judge Emmet Sullivan in Washington thwarted the $6.3 billion deal late Tuesday, a victory for the Federal Trade Commission, which argued that uniting the national suppliers of pens and printer paper would harm buyers. The companies said they wouldn’t appeal the ruling and would terminate their agreement effective May 16.

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