The contract covering some 70,000 members of the United Food and Commercial Workers Union at three major supermarket chains -- Kroger, Albertsons Inc. and Safeway Inc. -- was approved by an 86 percent margin, a spokeswoman for the union said.

Almost 900 stores were affected by the strike and lock-out, estimated to have cost the supermarkets more than $1 billion in lost sales.

The 20-week-long labour dispute centred largely on health care costs, with supermarket chains saying they could no longer afford to pay for the benefits without contributions from workers in the face of competition from non-union rivals like Wal-Mart Stores Inc.

Union leaders and the supermarkets reached a tentative agreement on Thursday following 15 days of intense talks.

Union members voted by secret ballot over the weekend at makeshift polling locations set up by union officials at school gymnasiums, union halls, convention centres and hotel conference centres throughout Southern California. A majority vote was required for ratification.

Workers were expected to return to work this week.

Under the terms of the three-year agreement, current union members will not have to make any contributions towards their health care plans in the first two years and will only need to pay from $5 to $15 in the third year if health care contributions in reserves do not cover the costs, union spokeswoman Ellen Anreder said.

However, the contract creates a second, lower tier of supermarket employees who will receive less pay and inferior benefits. New supermarket hires will have to pay about $9 a week for a basic health care plan and will make less than the average wages of $12 to $14 an hour earned by their veteran counterparts.