Wal-Mart has come under renewed pressure to recognise unionised shopworkers, following the decision by a US court to grant class action status to a sex discrimination case against the retailer.
Leaders of some of the largest storeworkers' unions in the US - including the United Food and Commercial Workers (UFCW) and the Service Employees International Union (SEIU) - claim the lawsuit has significantly boosted their efforts to organise Wal-Mart employees.
The move also weakens Wal-Mart's long-standing anti-union stance.
The SEIU has revealed a US$1 million (£544,700) effort to organise Wal-Mart workers, while a UFCW spokesman predicted that the chain could face a fresh wave of litigation.
'Workers are much more likely to take action now than ever before, as this case shows. It would have been inconceivable 10 years ago for Wal-Mart workers to take action collectively in this way,' the spokesman said.
Wal-Mart has so far been successful in discouraging workers from organising.
'Our associates have rejected union proposals time and time again. We do not believe that unionisation is right for Wal-Mart,' said a company spokesman.
As the largest employer in the US, Wal-Mart sets the conditions by which its smaller rivals operate in order to remain competitive. 'All other companies in the sector are impacted by Wal-Mart's position,' said the UFCW spokesman.
Wal-Mart is appealing against the class action ruling, in which it potentially faces cases from about 1.6 million women - the largest prospective class action case in US history.