Rupert Murdoch
News Corp chief executive Rupert Murdoch joined Twitter in December

The world's most powerful media mogul, Rupert Murdoch, has launched a stinging attack on Republican frontrunner Mitt Romney, arguing that his controversial tax returns could kill his candidacy.

Murdoch, whose plethora of global media outlets includes the right-wing Fox News, tweeted: "Romney's tax returns might kill his chances. See Republican establishment panic now!"

Romney released two years of his tax returns to reveal a combined income of $43m (£27m) with his wife Ann but sparked controversy because the figures showed that he paid less than 13 percent income tax ­- a fraction compared to the average working American who pays up to 35 percent.

Murdoch's press has also been attacking the former governor of Massachusetts with the Wall Street Journal suggesting Romney's wealth embarrassed him.

The Journal also reported that Romney was thinking of scrapping the so-called "carried interest" provision that has allowed him and other venture capitalists to invest and reap profits at the low capital gains tax rate of 15 percent.

If "carried interest" were scrapped, rates on venture investments could be taxed at normal income rates.

Rival candidate Newt Gingrich has used the tax returns opportunistically for his own campaign and called for a flat 15 percent income tax for all Americans.

Gingrich said scrapping carried interest would be folly. Gingrich argued he would cut capital gains taxes across the board to zero to stimulate investment and create new jobs.

The Politico website said: "Mr Romney remains in a defensive crouch, as if he's embarrassed by his wealth."