Japan's Softbank Corp plans to announce a deal to acquire control of U.S. carrier Sprint Nextel Corp at a news conference at 0800 GMT, a source said on Monday.

Softbank's takeover of Sprint in an expected $20 billion deal would be Japan's biggest overseas buy, and would also give the U.S. firm ammunition to potentially acquire peers and build out its 4G network to compete better in a wireless market dominated by AT&T and Verizon.

(Reporting by Maki Shiraki; Editing by Muralikumar Anantharaman)