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WASHINGTON (Reuters) - The U.S. Department of Justice approved Exelon Corp's purchase of Constellation Energy Group Inc on the condition it divest three electricity generating plants in Maryland to preserve competition.

The department said it required the sale to prevent the combined companies from having too much clout in power production in the mid-Atlantic states once the $7.9 billion deal closes.

Two of the plants to be sold are in Maryland's Anne Arundel County and a third is in Baltimore County, Md.

"These divestitures will preserve that critical competition for the benefit of electricity customers throughout the mid-Atlantic," said Sharis Pozen, acting assistant attorney general for the Justice Department's Antitrust Division.

The planned acquisition was announced in April. The deal still requires approval by the Maryland Public Service Commission, the state utility regulator. (Reporting By Diane Bartz and Jim Vicini; Editing by Maureen Bavdek and Editing by Tim Dobbyn)