Three major Southern California grocery chains that are in contract talks with thousands of employees have agreed to lock out all their workers if employees target one chain with a strike.

The three chains Kroger Co.’s Ralphs, Safeway Inc.’s Vons and Supervalu Inc.’s Albertsons issued the joint statement just days before a contract extension expires on April 9. It follows a vote last week by Albertsons workers authorizing a strike if an agreement is not reached.

The agreement “calls for the lockout of employees from all three companies within 48 hours of a strike against any one company,” according to the joint statement.

“We signed this agreement to protect our companies, our customers and our employees’ jobs in the event of a union strike,” spokeswoman Adena Tessler said. “It is now clear that the unions are engaged in a unified negotiating and strike strategy designed to put pressure on one company to agree to competitive contract provisions and gain significant bargaining leverage against the other companies.”

Union leaders blasted the agreement, calling it a “shocking display of contempt” on the part of the management of Ralphs, Albertsons and Vons.

“With this announcement, the markets have very clearly said they would rather have a lockout or strike than compromise on a fair contract with their employees,” said Mike Shimpock, spokesman for the Southern California Grocery Workers Union.

The union is pushing for the repeal of a two-tier wage system that it agreed to at the conclusion of a devastating strike three years ago. The grocery chains have resisted the efforts to roll back the system in which new workers are paid less and receive fewer benefits than veteran employees.

That strike began when Ralphs locked out employees after grocery workers voted to strike against Vons and Pavilions.