Southwest Airlines Co. pilots spurned a tentative contract that would have raised pay and company retirement contributions, the second rebuff by a major labor group since July at the world’s biggest discount carrier.

The four-year proposal was rejected with 62 percent of the vote, the Southwest Airlines Pilots’ Association said in a statement Wednesday. The agreement was reached in September after more than three years of negotiations with the union representing 8,000 pilots.

The vote sends Southwest and its pilots back to the negotiating table as the airline seeks offset rising costs with guarantees of increased productivity in exchange for higher pay. Flight attendants turned down a contract proposal that would have extended their work days while providing a mix of raises and bonuses.

Southwest said pilots will continue working under their current accord until a new deal is reached. Mediated talks via the National Mediation Board probably will resume in spring 2016, Southwest said in an e-mailed statement.

“We are willing to meet with company management at any time to continue work toward a deal,” union President Paul Jackson said in the statement. The union’s board will meet next week to prepare for a return of negotiations.

Spending for labor, Southwest’s largest expense, rose 17 percent through September. The tentative contract would have put pilots’ pay “in line with other major airlines,” according to the union, which didn’t provide specifics. Union board members sent the accord to pilots without a recommendation for approval.

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