LOS ANGELES — United Airlines apologized on Wednesday for inadvertently reactivating the retired flight numbers of the two planes that crashed after being hijacked by terrorists on Sept. 11, 2001.

The flight numbers were assigned by a computer late Monday for sales of future flights, said Rahsaan Johnson, an airline spokesman. Early Wednesday, airline officials noticed the error and immediately removed them from the system.

“We did not make a decision to use those flight numbers,” Johnson said. “The error was technical.”

Although not required by the Federal Aviation Administration, as a practice airlines typically retire the numbers of flights involved in fatal crashes, an FAA spokeswoman said.

United Flight 93 was hijacked while en route to San Francisco from Newark, N.J., and crashed into Shanksville, Pa., killing all 44 people aboard. United Flight 175, traveling from Boston to Los Angeles, was hijacked and crashed into the south tower of the World Trade Center in New York with 65 people aboard.

Though the flights will be operated by Continental Airlines, they were given a United flight number under a code-share agreement between the two airlines. In October, United and Continental merged and became United Continental Holdings Inc. and are in the process of combining their flight operations.

Because the numbers were assigned for sales of future flights, the numbers did not appear on screens at airport check-in counters, departure/arrivals monitors or gates.

It is, however, possible that the numbers were displayed on airline and travel websites for reservation, Johnson said.

Johnson declined to comment on whether any tickets were sold with the flight numbers. If any were purchased, customers will receive a new itinerary and flight number, he said.

American Airlines also retired its flight numbers of planes used in the attacks on Sept. 11, 2001.