"The class is larger than the active-duty personnel in the army, navy, air force, marines, and coast guard combined - making it the largest employment class action in history by several orders of magnitude," Walmart argued in its petition to the court filed on Wednesday.

The corporation is arguing that the petition involves too many people to proceed as one case.

'Size unusual'

Brad Seligman, the lawyer representing six female employees who are spearheading the case, said Walmart's appeal was an attempt to deny clients their day in court.

"The ruling upholding the class in this case is well within the mainstream that courts at all levels have recognised for decades," he said.

"Only the size of the case is unusual, and that is a product of Walmart's size and the breadth of the discrimination we documented."

The case was authorised by a divided 9th Circut US Court of Appeals in April by a 6-5 decision.

The lawsuit was allowed to proceed on the grounds that its mere size should not be a reason to dismiss the case.

Lawyers for Walmart insisted that the women named in the case should sue individually and that the size of the class action, which could in theory include as many as 1.5 million women, made it impossible to defend against.

With more than 2.1 million employees, Walmart is the world's largest private employer and the litigation could involve billions of dollars in damages.