U.S. investment fund Cerberus Group will start formal procedures early this week to buy all the Aozora Bank shares held by its largest shareholder, Softbank Corp., through a tender offer, according to sources close to the deal.

Softbank holds approximately 49 percent of Aozora's issued shares and the purchase -- estimated at around 100 billion yen -- would raise Cerberus' stake in Aozora to more than 60 percent. The U.S. fund is expected to sell part of its holdings in the future and reduce its stake to less than 50 percent.

Cerberus plans to file a request with the Financial Services Agency this week to buy the Softbank stake. Shareholders owning more than 20 percent of the shares in a financial institution are subject to FSA scrutiny.

The FSA is likely to authorize the purchase in early August.

Softbank has said it is unloading its Aozora holdings in order to focus on the high-speed telecommunication business.