Under mounting pressure from rivals, US Republican White House candidate Mitt Romney released tax documents disclosing his multi-million-dollar income on Tuesday, according to the Washington Post.

Romney -- a former venture capitalist and among the richest individuals ever to run for president -- reported income of $21.7 million in 2010 and $20.9 million last year, virtually all of it from investments, the Post reported.

The Romney family has given away $7 million in charitable contributions since 2010 -- mostly to the Mormon Church -- and paid just $6.2 million in federal income taxes during the same period, the Post reported.

Romney paid $3 million in 2010 for an effective tax rate of 13.9 percent and estimates he will pay $3.2 million for his income last year, a rate of 15.4 percent -- far less than former House speaker Newt Gingrich, his chief Republican rival, or Democratic President Barack Obama.

Rival candidates have been hammering Romney -- a former Massachusetts governor and on-again off-again frontrunner -- to release his tax returns in line with other presidential candidates past and present.

Romney had hinted he would release them following the April filing deadline for 2011, but appeared to have changed course following his crushing loss to Gingrich in Saturday's South Carolina primary.

Romney's opponents -- both Democrats and Republicans -- have accused him of preying on companies during his years at the head of equity firm Bain Capital, reaping huge profits while gutting firms and erasing jobs.

Romney has said his business acumen makes him the candidate most able to steer the country out of the economic doldrums, arguing that he turned around several companies and generated tens of thousands of jobs while at Bain.

The Post said Romney had provided the newspaper with a preview of his 2011 and 2010 tax returns ahead of a planned public release later on Tuesday.

It said the nearly 550 pages of tax documents detail far-flung holdings in notorious tax havens like Luxembourg, Ireland and the Cayman Islands, but said none of the offshore accounts showed much income.

It quoted a campaign spokesman as saying that Romney had paid all the taxes required on all of his income, both domestic and overseas.

And at a debate last week Romney stood by his earnings, saying: "I know the Democrats want to go after the fact that I've been successful. I'm not going to apologize for being successful."

Romney and Gingrich are locked in an bitter struggle over delegate-rich Florida, the next state to vote in the months-long process to select a Republican to challenge Obama in November.

Under US tax law income from investments is usually taxed at 15 percent while wages -- the main source of income for a majority of Americans -- can be taxed at up to 35 percent.

Obama and his Democratic Party have argued that millionaires and big corporations should pay more in taxes to help trim the country's massive debt, while Republicans oppose any tax hike, saying it would stunt job creation.

The country's growing economic inequality promises to be a major issue in the general election, with Republicans blaming Obama for the sluggish economic recovery and Obama accusing his rivals of ignoring the struggling middle class.