Wealth inequality around the world is "out of control" and doing particular harm to women, anti-poverty campaigner Oxfam warned ahead of the annual gathering of business and political leaders in the Swiss resort of Davos.

Oxfam, which brings attention to the issue ahead of the World Economic Forum, said that billionaire fortunes increased by 12 per cent last year - US$2.5 billion a day - while the 3.8 billion people who make up the world's poorest half saw their wealth decline by 11 per cent.

"This is not inevitable, this is unacceptable," Winnie Byanyima, Oxfam International's executive director said.

In the report, which is based on figures from Credit Suisse' Wealth Databook and Forbes' annual list of billionaires, Oxfam said the number of billionaires has almost doubled since the financial crisis a decade ago yet tax rates on the wealthy and corporations have fallen to their lowest levels in decades.

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"While corporations and the super-rich enjoy low tax bills, millions of girls are denied a decent education and women are dying for lack of maternity care," Byanyima said.

Oxfam said making taxes fairer will help address many of the world's ills. It said getting the world's richest 1 per cent to pay just 0.5 per cent extra tax on their wealth could raise more money than it would cost to educate the 262 million children out of school, and provide lifesaving healthcare for 3.3 million people. It also suggested governments look again at taxes on wealth such as inheritance or property.

- AP