The Mitt Romney campaign published details this morning of the Republican Presidential candidate's federal tax returns, showing that he expects to pay $6.2 million (£4m) in taxes on income of $45 million (£29m) from the last two years -- tax rates of 13.9 per cent in 2010 and 15.4 per cent in 2011.

The disclosure reveals the extent of Romney's wealth, questions about which have dogged his nomination campaign in recent weeks. Romney and his wife Ann hold around a quarter of a billion dollars in assets, largely derived from Romney's involvement in the private equity firm, Bain Capital. The Washington Post and other newspapers this morning reported the Romneys have a large numbers of offshore investments -- in parts of the world including Bermuda and the Cayman Islands -- with funds from a recently closed Swiss bank account.

The Romneys' incomes of $21.6m in 2010 and $20.9m in 2010 came mainly from investments, which under the US capital gains law are taxed at 15 per cent. The maximum tax rate on earned income is 35 per cent.

At a debate in Florida last night Romney said:

I pay all the taxes that are legally required and not a dollar more. I don't think you want someone as the candidate for president who pays more taxes than he owes.

The former Massachusetts governor noted that under rival Newt Gingrich's proposal to reduce capital gains taxes to zero, "I'd have paid no taxes in the last two years."

The Gingrich campaign made a surprising surge in recent weeks; the former Speaker of the House opened up the nominee race with a landslide win in the South Carolina primary. Fifty delegates are at stake on 31 January when four million registered Republican voters will take to the polls in Florida, choosing between the remaining candidates Romney, Gingrich, Rick Santorum and Ron Paul.