FORTY US billionaires have now pledged to give away at least 50 per cent of their wealth to charity as part of a campaign by investor Warren Buffett and Microsoft founder Bill Gates.

Among the billionaires joining the campaign are New York Mayor Michael Bloomberg, entertainment executive Barry Diller, Oracle co-founder Larry Ellison, energy tycoon T Boone Pickens, media mogul Ted Turner, David Rockefeller and investor Ronald Perelman, according to an announcement from The Giving Pledge campaign yesterday.

The Giving Pledge was started in June by Gates, whose $53 billion fortune places him second on the Forbes magazine list of the world's richest people, and Buffett, who ranks third on the list.

They wanted to persuade hundreds of US billionaires to give away most of their fortune during their lifetime or after their death and to publicly state their intention with a letter of explanation.

"We've really just started, but already we've had a terrific response," Mr Buffett said yesterday. "The Giving Pledge is about asking wealthy families to have important conversation about their wealth and how it will be used.

"We're delighted that so many people are doing just that - and that so many have decided to not only take this pledge but also to commit to sums far greater than the 50 per cent minimum level," he said.

Mr Buffett decided in 2006 to give 99 per cent of his fortune to charity. Then, he was worth about $44bn. After five years of investment returns while making annual gifts to five foundations, Mr Buffett's fortune totals nearly $46bn.

Mr Buffett said he and Bill Gates will also meet with groups of wealthy people in China and India within the next six months to talk about philanthropy. They hope the idea of generosity will spread, but they have no plans to lead a global campaign, Mr Buffett said.

The Giving Pledge asks billionaires to make a moral commitment to give away their wealth to charity.

"I am enthusiastically taking the Giving Pledge, and nearly all of my net worth will be given away in the years ahead or left to my foundation," Mr Bloomberg wrote in his Giving Pledge letter.

"Making a difference in people's lives - and seeing it with your own eyes - is perhaps the most satisfying thing you'll ever do."

"I've always thought your kids get more benefit out of your philanthropy than your will," he added.

Mr Bloomberg, Mr Bloomberg, founder of the Bloomberg financial data company, has a fortune estimated by Forbes magazine at $18bn

The billionaires announcing their pledge yesterday join real estate and construction billionaire Eli Broad, venture capitalist John Doerr, media entrepreneur Gerry Lenfest and former Cisco Systems chairman John Morgridge, who have already committed to giving away most of their wealth.

Mr Buffett, who made his fortune with insurance and investment company Berkshire Hathaway, Mr Gates and his wife Melinda held several dinners with a few dozen rich Americans in the past year to urge them to make the pledge.

Bill and Melinda Gates have so far donated more than $28bn of their fortune to the foundation.

Since the foundation began in 1994, it has given away more than $22bn for health improvements in poor countries and to improve access for Americans to opportunities they need to succeed in school and life.

"I've long stated that I enjoy making money, and I enjoy giving it away," Mr Pickens said in his Giving Pledge letter.

"I'm not a big fan of inherited wealth. It generally does more harm than good."

Mr Gates and Mr Buffett estimate their efforts could generate $600bn in charitable giving.

Forbes said the US is home to 403 billionaires, the highest concentration in the world.