Warren Buffett said he is stepping down as a trustee of the Bill and Melinda Gates Foundation and that he has donated half his wealth since pledging 15 years ago to give away the fortune he built up running Berkshire Hathaway.
Mr Buffett, 90, said he has been an "inactive trustee" for years at the foundation, but fully supported its new chief executive and that their goals remained "100 per cent in sync".
The nonprofit's future has been uncertain following last month's announcement that co-founders Bill Gates and Melinda French Gates were divorcing after 27 years of marriage.
Mr Buffett did not mention the divorce as a reason for his resignation, noting he has given up all directorships outside Berkshire, reducing his workload.
The Gates Foundation did not immediately respond to a request for comment.
Mr Buffett donated $4.1 billion of Berkshire shares to the Gates Foundation and four family charities on Wednesday, part of his 2006 pledge to donate about 99 per cent of his net worth to philanthropy.
He has since donated more than $41.5bn of Berkshire shares, a sum now equal to about $100bn, as the stock price has risen.
The Gates Foundation received four fifths of the donations. Founded in 2000, it focuses on combating poverty, disease and inequity, spending $54.8bn on such projects in its first two decades.
Arguably the world's most famous investor, Mr Buffett has since 1965 built Omaha, Nebraska-based Berkshire into a more than $600bn conglomerate, owning businesses such as the BNSF railroad and Geico auto insurance as well as stocks such as Apple.
"Over many decades, I have accumulated an almost incomprehensible sum simply by doing what I love to do," Mr Buffett said. "Society has a use for my money; I don't."
Wednesday's donations also go to the Susan Thompson Buffett Foundation, named after Mr Buffett's late first wife, and charities run by Mr Buffett's children Howard, Susan and Peter: the Howard G Buffett Foundation, the Sherwood Foundation and the Novo Foundation.
Had Mr Buffett not made his donations, his fortune would roughly equal that of Amazo founder Jeff Bezos, the world's richest person, according to Forbes magazine.
Mr Gates and Mr Buffett also pioneered "The Giving Pledge", through which more than 200 people, including Michael Bloomberg, Larry Ellison, Carl Icahn, Elon Musk and Mark Zuckerberg, committed at least half their fortunes to philanthropy.
Mr Buffett's statement also addressed recent criticism after a ProPublica investigation found that he and other wealthy people paid low taxes relative to their fortunes.
He said his donations have resulted in only about 40 cents of tax savings per $1,000 given.
"Nevertheless, tax deductions are important to many – particularly to the super-rich – who give large amounts of cash or securities to philanthropy," he said.
Mr Buffett also used references to American football to assure fans he has no plans to step away, including from Berkshire.
"These remarks are no swan song," he said. "I still relish being on the field and carrying the ball. But I'm clearly playing in a game that, for me, has moved past the fourth quarter into overtime."
