Updated from 1:58 p.m. EDT

United Airlines

has canceled thousands of flights during one of its busiest seasons because many of its pilots are refusing to work overtime.

A United spokesman confirmed Friday that the airline, the world's largest, had to cancel 4,800 flights scheduled between May and September after members of the

Air Line Pilots Association

refused to fly overtime. United, which is owned by Elk Grove Township, Ill.-based

UAL

(UAL) - Get Report

, operates about 2,500 flights a day.

The pilots' union blames the airline for scheduling more flights than there were crews to fly them. While United offers its pilots opportunities to work overtime, the pilots are not required to do so.

"United is trying to correct a mistake they made -- they created too many flights," said Herb Hunter, a union spokesman for the pilots' union and a pilot for more than 20 years. "We told them over a year ago that we had too many flights and not enough people. The airline told us 'You guys fly the airplanes, we'll run the airlines.' Now they're sort of caught with their pants down."

The United Airlines spokesman, Matt Triaca, said the canceled flights represent less than 2% of the total number of flights scheduled for the summer season. He added that the flights were canceled in April, after some of the union pilots announced they would no longer work overtime.

The United pilots' labor contract became amendable on April 12, but the two groups are still negotiating a new contract. The airline employs about 11,000 pilots.

Hunter said some of the cancellations were also likely a result of bad weather in the Midwest. He said he was unsure how many United pilots had refused to work overtime and emphasized that those who had, were not cutting their shifts short because they were about to go on the overtime clock. "It just means not coming in on their extra days off," he said.

The flight cancellations were first reported Friday in the

Washington Post

.

Wall Street showed little reaction to the news. UAL closed down 3/8, or 0.7%, at 57 11/16.

The pilots' union is in the midst of negotiating a new labor contract with United, a process that has been complicated by the airline's proposal to buy

US Airways

(U)

. If the deal is approved by shareholders and federal regulators, the two airlines would merge their pilot seniority lists. The United Airlines pilots have come out against the current terms of the proposed merger, but say they could support the deal if issues surrounding pilot seniority and other labor-related matters are resolved.

The Air Lines Pilots Association, represents approximately 10,000 United pilots who own approximately one-quarter of UAL's stock. United Airlines has been employee-controlled since mid-1994, and the airline's employees remain majority shareholders

The airlines and union leaders are scheduled to meet next in Chicago from July 24 to July 28 to continue face-to-face negotiations. Hunter said he's been encouraged by recent comments from James E. Goodwin, United Airlines' chairman and chief executive, expressing a desire to draw up a contract by the end of the month.

Triaca did not offer a timeline for resolving the current situation, but said the airline hoped to reach an agreement with the pilots "as soon as possible." He added that no additional cancellations have yet been planned for the fall season schedule.

The United spokesman added that the airline is still in the process of contacting those customers who had booked seats on the flights that have been canceled. It has already sent a letter to members of its Mileage Plus and Premier frequent-flier clubs, apologizing for any inconvenience they might have experienced as a result of shifts in scheduling and cancellations.

Many of the flights canceled, the equivalent of about two days' worth, were part of the "wing tip flying" schedule -- flights departing within minutes of each other headed for the same destination, said Triaca. When possible, the airline would cancel flights involving small airplanes, substituting instead a larger jet that could accommodate nearly as many passengers as the two smaller jets combined.