WASHINGTON  The Supreme Court on Monday agreed to hear an appeal in the biggest employment discrimination case in the nation’s history, one claiming that Wal-Mart Stores had discriminated against hundreds of thousands of women in pay and promotion. The lawsuit seeks back pay that could amount to billions of dollars.

The question is not whether there was discrimination but rather whether the claims by the individual employees may be combined as a class action. The court’s decision on that issue will almost certainly affect all sorts of class-action suits, including ones asserting antitrust, securities and product liability.

If nothing else, many pending class actions will slow or stop while litigants and courts await the decision in the case. Arguments in the case are likely to be heard this spring, with a decision expected by the end of June.

Wal-Mart, which says its policies expressly bar discrimination and promote diversity, said the women in the potential class action  who worked in 3,400 stores in 170 job classifications  could not possibly have enough in common to make class-action treatment appropriate.