TOKYO (Reuters) - Shares of Softbank Corp <9984.T> slid more than 5 percent to a five-month low on Monday after a source said the Japanese mobile carrier is close to securing a $20 billion deal to acquire control of U.S. counterpart Sprint Nextel Corp .

Softbank shares were down 5.3 percent at 2,267 yen and were the most heavily traded share by turnover on the main board.

The deal, which could come as soon as Monday, is expected to be structured in two parts, with Softbank providing about $8 billion of new capital and acquiring an additional $12 billion worth of shares from Sprint shareholders, a source familiar with the matter said.

(Reporting by Mari Saito; Editing by Michael Watson)