The Members Exchange (MEMX), a new U.S. stock exchange backed by some of Wall Street's biggest firms that is preparing for a Sept. 4 launch, said it received financial backing from Citigroup Inc.

"Although we recently closed our latest round of financing and are focused on launch, we didn't want to pass up the opportunity to add another significant market participant to our list of supporters," MEMX Chief Executive Officer Jonathan Kellner said on the company's website late on Thursday.

(Reporting by John McCrank; Editing by Chizu Nomiyama)