A global minimum tax on billionaires, equal to 2% of their wealth, could raise nearly $250 billion a year, according to a think tank co-funded by the European Union.

The EU Tax Observatory proposed such a tax in a report released Sunday and said it would affect fewer than 3,000 billionaires. Billionaires have effective tax rates equivalent to zero to 0.5% of their wealth because they use shell companies to dodge income taxation, said the think tank, which is located at the Paris School of Economics.