Gain Capital Holdings Inc, an online provider of retail foreign exchange trading services, is planning to raise up to $125 million in the public markets, according to a regulatory filing on Monday.

Gain Capital, based in Bedminster, New Jersey, operates the website which, for the six months ended June 30, averaged 1.3 million unique visitors per month, according to an initial public offering prospectus filed with the U.S. Securities and Exchange Commission.

Gain Capital, which was founded in 1999 by Wall Street traders, said in the filing that 56.7 percent of its clients were based outside the United States, including until last year China, which in December 2008 represented 26.8 percent of its clients and about 11.6 percent of total trading volume.

But the company abruptly abandoned the Chinese market in December after a regulatory review by the government there, and its revenues fell.

For the six months ended in June 2009, Gain Capital's net revenues fell 12.2 percent from a year earlier to $77.8 million, with a net loss of $48.7 million. The company had net income of $135.3 million in the year-ago period.

According to the filing, Gain Capital had to leave the Chinese market after regulators in 2008 found it did not have a license to provide retail foreign exchange trading services to Chinese residents through the Internet. The company said in the filing it did not know such a permit to exist.

All of the IPO's proceeds will go to Gain Capital's existing shareholders, a potential impediment to investor interest in the offering, given a preference to see IPOs used at least partially to fund company growth.

The foreign exchange company has arrangements with three prime brokers -- Deutsche Bank, Royal Bank of Scotland PLC, and UBS AG -- to provide liquidity to its clients, it said.

The IPO will be led by Morgan Stanley and Deutsche Bank Securities.

Gain Capital plans to list its shares on Nasdaq.

(Reporting by Phil Wahba; Editing by Richard Chang, Phil Berlowitz)