Human Rights Watch has accused Wal-Mart, the US's largest private sector employer, of using anti-union tactics that violate the rights of its 1.3m US workers in a report that delivers another blow to the retailer's efforts to improve its reputation.

In "Discounting Rights", the international rights group set out what it calls "the sheer magnitude and aggressiveness of Wal-Mart's anti-union apparatus and actions" in thwarting organising drives by the UFCW grocery workers' union at its stores.