(REUTERS) - Warren Buffett this week donated US$2.8 billion (S$3.5 billion) of Berkshire Hathaway Inc stock to five charities, as part of the billionaire's plan to give away nearly all of his wealth.

Buffett donated about 21.73 million Class "B" shares of Berkshire on Monday, a regulatory filing shows.

About 16.6 million shares, worth more than US$2.1 billion, went to the Bill and Melinda Gates Foundation, which focuses on education, health and poverty problems.

The remainder went to four family charities, including a foundation named for his late first wife Susan and foundations for his children Howard, Peter and Susan.

The US$2.8 billion is the most Buffett has donated to these charities, reflecting Berkshire's near-record share price, since he began making annual gifts in 2006.

Buffett still controls nearly 20 per cent of Berkshire's stock, which made him worth US$65.8 billion on Monday, according to Forbes magazine.

Only Mexico telecommunications mogul Carlos Slim and Bill Gates, the Microsoft Corp co-founder and Berkshire director, were worth more, Forbes said.

Berkshire has more than 80 operating businesses and ended March with more than US$146 billion of stock and bond investments. Buffett has run the Omaha, Nebraska-based company since 1965.