Almost one in ten people earning over £10m per year pays less than the 20% basic rate of income tax, according to Treasury figures.
Data released by the government showed that 6% of £10m-plus earners paid less than 10% in tax while another 3% paid less than 20%.
Fewer than three-quarters of high earners paid more than 40% in income tax, according to figures quoted by the BBC.
"There are currently millionaires paying a lower tax rate than ordinary taxpayers," a Treasury spokesperson said.
The Treasury said it released the figures to show that government plans were needed to prevent very wealthy people from paying less tax than low-earners.
Existing proposals include limiting the tax relief on charitable donations.
Under the plans, previously uncapped tax reliefs - including on charitable donations - would be capped at £50,000, or 25% of a person's income, if that was higher, from 2013.
But that idea has been criticised by Labour and others, who argue it could put some people off giving large sums to good causes.
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