Five banks, including Citigroup, Lehman Brothers Holdings Inc., and Merrill Lynch & Co. Inc., said on Tuesday they were setting up a trading platform for privately offered stocks, seeking to tap the growing market for non-public shares.

The banks, which also include Morgan Stanley and the Bank of New York Mellon, said the platform is designed to ease trading for investors and allow greater access to capital for issuers. It is expected to be launched in September.

The platform is called Open Platform for Unregistered Securities, or OPUS-5, and will allow the firms to trade equities issued under rule 144a.

Goldman Sachs Group Inc., launched a 144a trading platform of its own in May. Private equity firm Apollo Management LP is planning to list on the platform known as Goldman Sachs Tradable Unregistered Equity system, or GSTrUE, according to press reports.

Last year, some $162 billion of unregistered shares were sold in the United States 144a market, outpacing the $154 billion raised in public offerings, according to Nasdaq Stock Market.

Along with the rise of hedge funds and private equity firms, 144a shares signal the trend of investors' capital moving out of regulated public markets and into unregulated private ones.

(Reporting by Dan Wilchins)