After-tax incomes for the richest 1 percent of the U.S. population soared by 275 percent on average between 1979 and 2007, while the poorest fifth saw a gain of only 18 percent, a study by the nonpartisan Congressional Budget Office shows.

The middle class, which constitutes 60 percent of Americans under the definition used by the CBO, experienced a 40 percent increase in income over the period.

"(T)he distribution of after-tax household income in the United States was substantially more unequal in 2007 than in 1979," the CBO said, noting that the share of total household income going to the top percent more than doubled over the 28-year span, from 8 percent to 17 percent.

Release of the report comes more than a month into the Occupy Wall Street protests in New York and other major cities, a movement highlighting the growing gap between the top 1 percent and everyone else.

The CBO analysis also coincides with debates in Washington over how to spur job creation as unemployment remains at 9.1 percent more than two years after the recession officially ended.

President Barack Obama has proposed a $447 billion jobs bill that envisions rolling back the Bush administration's big tax cuts for the wealthy as a way to pay for measures to stimulate the economy.

Republicans in Congress - and those vying for the GOP nomination to challenge Obama in 2012 - are adamantly imposed to boosting taxes.

Investor Warren Buffett, ranked by Forbes as the third-richest person on the planet, has been campaigning for an end to various provisions of the tax code that favor the wealthy.