In celebration of his eighty-second birthday, Warren Buffett, the investment legend, announced he is doubling his original pledge to his children’s charities - a gift worth an extra $3 billion in shares.

Back in 2006, Buffett pledged 17,500,000 Berkshire Hathaway  B shares to each of his children’s foundations: The Howard G Buffett Foundation, The Sherwood Foundation and The NoVo Foundation, established by Howard Buffett, Susan Buffett and Peter Buffett respectively. Warren Buffett is chairman and chief executive of Berkshire.

“I knew you would apply your considerable brains and energies in order to make the most of the funds from my gift,” he wrote to them in a letter yesterday. "However, you have exceeded my expectations," he added.

Warren Buffett’s original pledge now stands at 12,220,852 B shares, but on the new base of 24,441,704 pledged shares, he said he will distribute 1,222,085 shares to each of the three foundations next July, with that amount decreasing at 5 per cent per year subsequently.

“Over time, I would expect the value of the annual distribution to average more than $100 million, though it will vary substantially from year to year,” he explained in the letter.

According to the 2011 annual report to shareholders, 98 per cent of Warren Buffett's net worth is in Berkshire stock. In February, it emerged that he had chosen a successor as chief executive of the firm, although the individual has yet to be named.

Buffett, whose extraordinary talent in investment has netted him a worth of some $44 billion, also in 2006 made a large pledge to the Bill & Melinda Gates Foundation. As such, he earmarked 10 million B shares for BMG contributions, 5 per cent of which would be contributed each year for its benefit. He has also signed the Giving Pledge, which invites the wealthiest Americans to leave significant proportions of their estate to charity and has been signed by such diverse and illustrious names as George Lucas, Michael Bloomberg, David Rockefeller and Mark Zuckerberg.