Flight attendants at United Continental Holdings , formed from the merger of United and Continental airlines, have selected the Association of Flight Attendants to bargain for their labor contracts, the AFA union said on Wednesday.

The vote was a defeat for the International Association of Machinists, which represented the flight attendants at Continental Airlines, and marks a step toward integration of the carriers' work forces.

The AFA, which represented flight attendants at the old United Airlines, won the election with 55.1 percent of the vote. The union now represents about 25,000 active and retired flight attendants at the company.

The vote results must be certified by the National Mediation Board.

This election is an historic moment for AFA and our profession, said Veda Shook, AFA International president, in a statement.

United and Continental closed their $3.17 billion merger in 2010, forming the world's largest airline, which is known as United Airlines. The carrier is trying to integrate its two work forces and achieve joint labor contracts.

The IAM-represented flight attendants at Continental ratified a contract on February 25. The AFA-represented flight attendants from the old United are in mediated contract negotiations with management.

With the union representation election complete, the AFA will attempt to win a joint contract for both groups of flight attendants.

This is another important step forward in our integration process. We will continue to focus on reaching combined agreements for our flight attendants, United Continental said in a statement.

United Continental has about 87,000 employees that it must finish blending into one work force. The company aims to realize more than $1 billion in annual revenue and cost savings by 2013.

Shares of United Continental were down 44 cents at $22.84 in afternoon trade on the New York Stock Exchange.

(Reporting by Kyle Peterson; Editing by Robert MacMillan and John Wallace)

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