The foundation created by billionaire Bill Gates and his wife Melinda plans to sell 60 million Class B shares of Berkshire Hathaway Inc (BRKb.N) donated by Warren Buffett, reflecting the fellow billionaire's desire that proceeds be spent on charitable works.

In a regulatory filing on Tuesday, the Bill & Melinda Gates Foundation said the sales would occur from July 1, 2017 to June 30, 2020, under a plan similar to one expiring on June 30, 2017.

Sixty million Class B shares of Berkshire are currently worth about US$9.6 billion.

The Gates Foundation, which works to improve education and health and reduce poverty worldwide, is the largest beneficiary of Buffett's 2006 commitment to donate nearly all of his wealth to charity.

The foundation, which received more than US$2.1 billion of Berkshire stock last year, said it owns about 68.71 million Class B shares, worth roughly US$11 billion.

Bill Gates is a co-founder of Microsoft Corp and a director of Berkshire. Buffett is also donating Berkshire shares to four family charities.

In the HBO documentary "Becoming Warren Buffett" scheduled to air on Jan. 30, Buffett explained why he is giving away his fortune, estimated on Wednesday by Forbes magazine at $71.6 billion despite more than US$24.3 billion of donations so far.

"In my entire lifetime, everything that I've spent will be quite a bit less than 1 per cent of everything I make. The other 99 per cent plus will go to others because it has no utility to me," Buffett said. "So it's silly for me to not transfer that utility to people who can use it."

The Gates Foundation will sell its Berkshire shares through a 10b5-1 program, named for a federal rule allowing periodic share sales by executives and other insiders without raising the specter of insider trading.