SoftBank already owns a majority of Sprint, U.S. ' third-largest carrier. T-Mobile is majority owned by Deutsche Telekom.
The Wall Street Journal had reported earlier this month that Sprint was studying regulatory concerns about such a deal and might make an offer for T-Mobile in the first half of next year.
SoftBank might make its offer as early as spring 2014, Nikkei said, roughly matching the earlier report on timing.
Combining Sprint and T-Mobile would create a carrier with nearly 100 million customers, close to subscriber parity with AT&T and Verizon Wireless, each of which has more than 100 million. However, U.S. regulators might block such a transaction in order to preserve competition in the the U.S. wireless industry.