Softbank will have to pay Sprint Nextel more than 1 trillion yen ($12,75 billion), the report added.
"The story about SoftBank and Sprint Nextel Corporation being reported is based on speculation," Softbank said in a statement. "We have not announced anything. We do not comment on speculation."
On the other hand, Sprint confirmed it is negotiating with Softbank regarding a "potential substantial investment by Softbank in Sprint.
Softbank announced earlier this month it will acquire smaller Japanese mobile phone service operator eAccess Ltd. through exchange of shares, increasing its number of total subscribers to 34 million in Japan.
The report of the Softbank-Sprint talks comes some days after T-Mobile and MetroPCS announced that they have agreed to combine their cellphone businesses.