Sprint Corp. is working toward a possible bid for rival T-Mobile US Inc., people familiar with the matter said, setting the stage for a giant telecom merger that if permitted by regulators would leave the U.S. wireless market dominated by three big companies.

Sprint is studying regulatory concerns and could launch a bid in the first half of next year, the people said. A deal could be worth more than $20 billion, depending on the size of any stake in T-Mobile that Sprint tries to buy.

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