Thirty-eight US billionaires have pledged at least 50% of their wealth to charity through a campaign started by investor Warren Buffett and Microsoft founder Bill Gates.

They include New York Mayor Michael Bloomberg, CNN founder Ted Turner and entertainment executive Barry Diller.

"The Giving Pledge" lists all the families and individuals who have committed to the project.

The site says the pledge is a "moral commitment" not a "legal contract".

The campaign was started in June to convince US billionaires to give away at least half of their fortunes either during their lifetimes or after their deaths.

"We've really just started but already we've had a terrific response," Mr Buffett said in a statement.

He added: "The Giving Pledge is about asking wealthy families to have important conversations about their wealth and how it will be used."

Those who pledge their money to "philanthropic causes and charitable organisations" must publicly state their intention through a letter of explanation.

'Improving education'

Other billionaires who have pledged large sums of their money include film producer George Lucas, philanthropist David Rockefeller and oil investor T Boone Pickens.

"I am dedicating the majority of my wealth to improving education. It is the key to the survival of the human race," Mr Lucas wrote in his Giving Pledge letter.

The organisation says many of the donors have committed to donating sums far greater than the 50% minimum level.

"While the Giving Pledge is specifically focused on billionaires, the idea takes its inspiration from efforts in the past and at present that encourage and recognize givers of all financial means and backgrounds," says Givingpledge.org.

I've long stated that I enjoy making money, and I enjoy giving it away T Boone Pickens, Oil investor

Mr Buffett along with Mr Gates and his wife, Melinda, held numerous dinners with US billionaires in the past year to promote the campaign and urge America's financial elite to pledge.

"I've long stated that I enjoy making money, and I enjoy giving it away. I like making money more, but giving it away is a close second," Mr Pickens wrote in a letter on Givingpledge.org.

Mr Buffett, the chief executive of the investment firm Berkshire Hathaway, pledged 99% of his money to the Bill & Melinda Gates Foundation and family charities in 2006.

Forbes Magazine estimated Mr Buffett's wealth at $47bn (£29bn) in March.

Bill Gates, who Forbes rates as the world's second wealthiest person, has also given away more than $28bn to his foundation.

There are 403 billionaires living in the US, according to Forbes.

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