Owners of Apple Inc.'s iPhone may have unlocked an "astounding" 1 million handsets to run on unauthorized wireless networks, depriving the company of lucrative monthly fees, Sanford C. Bernstein & Co. said.

That estimate represents 27 percent of about 3.75 million iPhones sold last year, Toni Sacconaghi, the top-ranked computer analyst by Institutional Investor magazine, said Monday in a note.

Apple gets an undisclosed cut of monthly service fees for the iPhone, which is typically sold with two-year contracts from wireless providers in the United States and Europe. AT&T Inc. is Apple's exclusive wireless partner in the United States. Widespread unlocking may make it more difficult for Cupertino, Calif.-based Apple to sign revenue-sharing deals with new carriers, Sacconaghi said.

Apple spokeswoman Natalie Kerris declined to comment.