Romney is one of the wealthiest US presidential candidates in history [GALLO/GETTY]

Mitt Romney, the Republican US presidential candidate, has said that he will release his tax returns for the last two years this week.

Sunday's announcement comes as the one-time front-runner admitted talk surrounding his returns hurt him in this weekend's primary in South Carolina, where he lost to former house speaker Newt Gingrich.

"I will release my tax returns for 2010, which is the last returns which were completed, on Tuesday of this week," Romney said on the Fox News network. The former Massachussetts governor also said he would release an estimate for the 2011 returns, officially due in April.

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"We made a mistake holding off as long as we did and it just was a distraction".

In a surprising upset, Gingrich won 40 per cent of the vote in Saturday's South Carolina primary, while Romney finished a previously unexpected second with 28 per cent.

Romney, who hails from a wealthy family and then went on to a financially successful tenure as co-founder of a private equity investment firm, is one of the wealthiest US presidential candidates in history.

In the announcement, he emphasized he was releasing two years of returns after Gingrich posted his returns for only 2010 on Thursday.

Gingrich and other Republican rivals attacked Romney in debates last week, asking why the former executive would not release his returns.

Romney had originally said he would release his records in April, when returns for all US tax-payers are due.

Romney's statement that he paid an approximate 15 per cent in taxes, below the national average for wage earners, but in line with those who derive wealth from investments, earned him criticism at a time when the economy is on the fore-front of American voters' minds.