Giant superstores such as those operated by Wal-Mart have been banned by the Oakland city council in an attempt to protect smaller outlets and their employees.

Councillors on Tuesday voted to ban any "big box" retailer with more than 10,000 square feet of non-taxable sales from opening in the city, which is located just across the bay from San Francisco.

The decision comes as Wal-Mart Stores Inc, the world's largest retailer, plans to roll-out around 40 super-centres in California over the next four years.