(Reuters) - U.S. budget airline Southwest Airlines Co said on Monday that its flight attendants union voted to ratify a tentative agreement that the parties had reached in September.

The agreement includes a ratification bonus and annual wage rate increases over the life of the contract, the airline said in a statement.

Southwest Airlines said the new contract would become amendable on Nov. 1, 2018.

The discussion between Southwest and the union, which represents more than 14,500 Southwest flight attendants, started in 2013.

The union had in July 2015 rejected a tentative contract that would have raised wages, but involved changes in work rules that many thought were burdensome.

Southwest Airlines' shares were up about 2 percent at their session highs in late afternoon trading on the New York Stock Exchange.

(Reporting by Arunima Banerjee in Bengaluru; Editing by Savio D'Souza)