Japan's Softbank Corp. <9984.T> is nearing a deal to buy a 70 percent stake in Sprint Nextel Corp for about $20 billion (12 billion pounds), a source familiar with the matter said.

The deal, which could come as soon as Monday, is expected to be structured in two parts: Softbank will provide about $8 billion of new capital and acquire an additional $12 billion shares from Sprint holders, the source said.

The No. 3 carrier behind Verizon Communications Inc and AT&T Inc Sprint has been struggling to keep market share over the years and faces a heavy debt burden between 2013 and 2014.

Sprint's $2.1 billion revolving credit facility expires in October 2013. The investment will help boost Sprint's capital requirements, the source said.

Sprint and Softbank spokespeople were not immediately available for comment.

CNBC's David Faber reported the news earlier on Sunday.

(Reporting By Nadia Damouni; editing by Gunna Dickson)