Legislation to make it easier for US workers to form unions was introduced in Congress yesterday, setting up a battle between business and employees' groups and sparking a broker downgrade to Wal-Mart, the world's biggest retailer.

Citigroup cut its target price on Wal-Mart - the largest US private sector employer and a regular target of union criticism - and downgraded its recommendation to "hold" on the belief that the retail giant could be forced to pay higher wages and benefits if the bill were made law.