Hewlett-Packard Co. (NYSE: HPQ), the biggest computer company, may have overbilled New York City by as much as $50 million to upgrade the 911 system, Comptroller John Liu charged.

While the overbilling could be as low as $2 million, Liu also disclosed the Palo Alto, Calif., company may have gotten as much as $113 million for unsatisfactory work performed over several years.

The 911 overhaul, proposed by Mayor Michael Bloomberg, an independent elected three times since 2001 as a Republican, has cost $2 billion so far compared with $1.3 billion in the budget.

Liu, a Democrat, charged HP may not have been qualified to do the work. HP officials said they would review the comptroller's audit  and added its work had been performed within budget.

Shares of HP fell 11 cents cents to $22.63 middayThursday.