The world's largest retailers have for the first time agreed on a unified set of workplace standards aimed at eliminating problems such as child labour and unpaid wages in their vast global supply chains.

Wal-Mart, Tesco, Carrefour and Metro - the world's four largest supermarket chains with more than $500bn (£258bn) in aggregate annual sales - have been working with Migros, the largest Swiss retailer, to develop a draft code of standards called the Global Social Compliance Programme.