The Chancellor has said the 50p top rate of income tax is only a "temporary" measure, though he is not yet ready to scrap it.
In his Budget speech to the House of Commons, George Osborne told MPs it is right the wealthy pay more in tax.
But he admitted a permanant 50% tax could do "lasting damage" to Britain's economy.
Right-wing think tank the Adam Smith Institute has said the top rate tax would cost the government £350bn and lead to flat economic growth over the next decade, as high-earners flee overseas and use tax avoidance to protect their income.
Britain is 83rd out of 86 leading economies on the level of its top income tax rate, with only the Netherlands, Denmark and Sweden taxing high-earners more, a report by the Institute said.
Two global vehicles
'Further plug advice gap'
Must appoint separate CEOs and boards
Advisers do come out well
Will report to Mark Till