IBM is facing an antitrust inquiry from the US Department of Justice over its mainframe business.

US authorities have begun issuing formal requests for information related to a complaint filed against IBM in September, according to the Computer and Communications Industry Association (CCIA), the trade group that filed the complaint.

CCIA's complaint against IBM alleges that the company has refused to issue licences for IBM's mainframe OS to competitors, as required in a series of actions the DoJ took against IBM dating back to the 1970s and earlier. In some cases, IBM has yanked the OS licence from customers trying to switch from IBM mainframe hardware to a competitor's, said Ed Black, CCIA's president.

The DOJ abandoned a long-standing antitrust consent decree with IBM in 2001.

Several large and small companies would like to compete with IBM in the mainframe market, particularly in software and services for mainframes, but IBM's actions have allowed the company to maintain a monopoly, Black said.

"It's a really big case," added Heather Greenfield, a CCIA spokeswoman. "There are tons of companies cheering that aren't willing to be quite so public because they have to deal with IBM in other ways."

The DOJ will not comment on the complaint, a DOJ spokeswoman said.

IBM believes there is no merit to antitrust claims brought by competitor T3 Technologies, said Tim Breuer, director of external relations for the IBM Systems and Technology Group. The DOJ has apparently asked T3 for documents related to its lawsuit, Breuer said. IBM intends to cooperate with any inquires from the DOJ, he added.

Last week, a US district court in New York dismissed a lawsuit brought by mainframe vendor T3, Breuer noted.