CHICAGO — For the first time in six years, all of United Airlines’ 25,000 flight attendants will be working under a single joint contract.

The Chicago-based airline’s flight attendants ratified the new contract Friday, with 53 percent voting in favor of the deal, the Association of Flight Attendants said.

“The contract provides immediate economic gains, sets a new industry standard and ensures flight attendants can achieve the benefits of a fully integrated airline,” Sara Nelson, the association’s president, said in the release.

The five-year agreement includes double-digit pay raises at all seniority levels, with top pay rates rising 18 to 31 percent, according to the union.

The deal also includes profit-sharing, continuation of a flight attendant-specific health care plan with new medical plan options, and improved retirement plans.

Flight attendants from United and Continental Airlines have been operating under different work rules since the airlines merged in 2010. That means they could only work with crew from the same legacy airline, on planes assigned to that airline, making scheduling or recovering from delays and cancellations more complicated.

The new contract will run roughly $1.9 billion over the five years — or about $380 million per year — above the combined cost of the current flight attendant contracts.

United CEO Oscar Munoz has consistently said labor relations are a top priority.

The new contract for the flight attendants follows separate deals the airline recently negotiated with pilots and dispatchers.

United employees including airport workers, security officers and other staffers also ratified a new contract in April, but the airline is still working on a pact with its mechanics.