By Jonathan Stempel

(Reuters) -Warren Buffett donated on Friday another $6 billion of Berkshire Hathaway stock to the Gates Foundation and four family charities, his biggest annual donation since he began giving away his fortune nearly two decades ago.

The donation of about 12.36 million Berkshire Class B shares boosted Buffett's overall giving to the charities to well over $60 billion.

He donated 9.43 million shares to the Gates Foundation; 943,384 shares to the Susan Thompson Buffett Foundation; and 660,366 shares to each of three charities led respectively by his children Howard, Susie, and Peter: the Howard G. Buffett Foundation, Sherwood Foundation and NoVo Foundation.

Warren Buffett still owns 13.8% of Berkshire's stock, based on reported shares outstanding.

His $152 billion net worth prior to Friday's donations made him the world's fifth-richest person, according to Forbes magazine.

Buffett would rank sixth after the donations, which surpassed the $5.3 billion he donated last June. He donated another $1.14 billion to the family charities last November.

In a statement, Buffett maintained he does not intend to sell any Berkshire shares.

Now 94, Buffett began giving away his fortune in 2006.

He changed his will last year, designating 99.5% of his remaining fortune after his death to a charitable trust overseen by his children.

They will have about a decade to distribute the money, and must decide where it goes unanimously. Susie Buffett is 71, Howard Buffett is 70, and Peter Buffett is 67.

Warren Buffett has led Omaha, Nebraska-based Berkshire since 1965.

The $1.05 trillion conglomerate owns close to 200 businesses including Geico car insurance and the BNSF railroad, and dozens of stocks including Apple and American Express.

Susie Buffett leads the Susan Thompson Buffett Foundation, which funds reproductive health and is named for her mother, who was Warren Buffett's first wife.

The Sherwood Foundation supports Nebraska nonprofits and early childhood education. The Howard G. Buffett Foundation focuses on global hunger, combating human trafficking and mitigating conflicts. The NoVo Foundation has initiatives focused on marginalized girls and women, and on indigenous communities.

Buffett said last June that donations to the Gates Foundation would end when he dies.

(Reporting by Jonathan Stempel in New York; Editing by Sandra Maler)