CHICAGO (AP) _ Union flight attendants have pulled out of a group of employees negotiating for a controlling stake in United Airlines.

The union said today that United acted in bad faith in opening a base for flight attendants in Taiwan, which will replace 200 job slots in this country.

The flight attendants, pilots and Machinists unions have been seeking to trade more than $3 billion in wage and benefit concessions for a majority stake in UAL Corp., United's parent company.

''We made it perfectly clear from day one that we wanted to be in this as long as United agreed not to open foreign domiciles,'' said Bobby Pilkington, secretary-treasurer of the attendant union's master executive council. ''United honored that commitment for three months, then without warning opened the base in Taipei.''

United spokesman Joe Hopkins said the airline ''remains fully receptive to continuing its discussions with the United Airlines union coalition.''

The unions represent 54,000 of the airline's nearly 83,000 employees. They have been seeking to buy United to head off a possible restructuring that could involve spinning off short-haul routes into a separate, lower-cost airline.

The Air Line Pilots Association told its members in a recorded telephone message that it and the Machinists union would continue the effort to buy United for their members.

But the message said the flight attendants' withdrawal makes the buyout effort more difficult and less certain.

Pilkington said the flight attendants still believe in the concept of employee ownership of the airline. She said the union has not decided what it would take to bring it back to the negotiating table.

The flight attendants said it was notified by United that the airline plans on opening other foreign bases, including one in Hong Kong and possibly others in Seoul and Manila.

This means that flight attendants living in these cities will be able to bid on international flights in their parts of the world, reducing the job openings for men and women in this country.

Pilkington said that 42 percent of the union membership flies on international flights, and that as these employees lose work, they will bid on domestic flights, bumping less senior employees out of their jobs.

The union represents 19,000 UAL flight attendants.

Flight attendants from this country can move to a foreign domicile and fly out of there and 55 employees already have transferred to Taipei, Pilkington said.

New flight attendants hired in foreign countries are required to join the union, just as new employees are in this country.

Pilkington acknowledged that the union won't be losing membership with foreign domiciles, but said it's responsibility is to its present membership, which will lose out.

Four previous employee efforts since 1987 to buy the airline have failed. The most recent was a $4.4 billion buyout offer by the three unions that was rejected by the company's board in 1990.

UAL has lost more than $1.4 billion in the past 2 1/2 years.