CHICAGO (AP) " United Airlines has tentatively negotiated a new contract for its 25,000 flight attendants, who will hold a ratification vote.

If approved, the agreement between United and the Association of Flight Attendants would let the airline mix cabin crews from United and Continental Airlines, which merged in 2010.

Terms of the contract were not disclosed.

United Continental Holdings Inc. and the union said Friday that negotiators were still working to finish contract language.

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The airline industry is heavily unionized. About 80 percent of United's employees are represented by unions, according to the company's latest annual report.

Shares of United Continental were down $2.56, or 5.7 percent, to $42.61 in midday trading. Airline stocks fell over uncertainty about how they will be affected when the United Kingdom exits the European Union.