NEW YORK (Reuters) - Billionaire investor activist Carl Icahn tweeted on Wednesday that he is forming a Super PAC with an initial commitment of $150 million, representing the biggest one-time injection of money in the history of such political action committees.

Icahn, who supports presidential candidate Donald Trump, said he is targeting "inversions," which occur when a company changes its domicile, often outside the United States, to take advantage of lower tax rates elsewhere.

"Right now, as we speak, there are many companies planning to leave this country," Icahn said in an interview with CNBC. "It's so simple to do something about it, it's a no-brainer."

Icahn said on CNBC that the incentive for companies to leave the U.S. via inversion deals could be eliminated by legislation allowing big companies to repatriate funds held offshore at a discounted tax rate - an approach also favored by Trump.

In a letter, released Wednesday and sent to several leading members of Congress, Icahn said as many as 50 companies have left the U.S. over the past few years, representing more than a half-trillion dollars in market value, costing hundreds of millions of dollars in taxes and lost jobs.

"There is zero reason not to allow major companies to repatriate this money," he said, adding that 12 to 15 lawmakers had responded to the letter so far.

"I believe my own commitment of $150 million to the PAC will be more than enough to make voters fully aware of the horrible consequences that will ensue if Congress fails to pass legislation immediately to stop these 'inversions,'" Icahn said.

Technically known as independent expenditure-only committees, super PACs may raise unlimited sums of money from corporations, unions, associations and individuals, then spend unlimited sums to overtly advocate for or against political candidates.

Icahn's pledge is even more than Jeb Bush's "shock-and-awe" fundraising haul of $100 million for his presidential bid. And it is half the $300 million amount that the Koch Brothers network, a group of wealthy mega donors founded by the billionaire industrialists, will pour into the 2016 campaign.

(Reporting By Jennifer Ablan and Michelle Conlin; Editing by Christian Plumb, Bernard Orr)