A report released Wednesday shows that more than 60 million workers in the United States are subject to mandatory arbitration in employment contracts, highlighting the potential scope of a key U.S. Supreme Court case that confronts the legality of those agreements.

The Economic Policy Institute's study conducted for the progressive think tank by a Cornell University professor shows the number of companies that use mandatory arbitration clauses in employment contracts has grown significantly since the 1990s.

The U.S. Supreme Court is preparing to hear oral arguments Monday in a series of consolidated cases that question the legality of clauses that limit the power of employees to join class actions against their employer. The consolidated cases before the justices are: National Labor Relations Board v. Murphy Oil USA; Epic Systems v. Lewis; and Ernst & Young v. Morris.

The data in the report found that in 1992, only 2 percent of workers required mandatory arbitration, as opposed to 55 percent in 2017. Additionally, the report found that 41 percent of employees subject to mandatory arbitration also waived their rights to be part of a class action.

“We knew they were subtly increasing, but this report shows the majority of private workers are subject of these kind of waivers,” said Celine McNicholas, labor counsel with the Economic Policy Institute. “It's a really scary concept.”

The rise in employee arbitration agreements has been driven by Supreme Court cases dating back to the 1991 case Gilmer v. Interstate/Johnson Lane, in which the justices upheld mandatory employment arbitration agreements. Other Supreme Court rulings—including AT&T Mobility LLC v. Concepcion and the 2013 case American Express v. Italian Colors Restaurants—found that these agreements were broadly enforceable.

In the upcoming case, set for argument on Oct. 2, the Supreme Court will review whether class action waivers and forced arbitration are violations of the National Labor Relations Act. The central issue is whether requiring the waiver infringes the collective speech protected by federal labor laws.

Dozens of major U.S. companies, with lawsuits on hold, are awaiting the outcome of the ruling. Just last week, a federal appeals court decided to postpone any ruling in a case that drivers for Uber Technologies Inc. brought against the ride-hailing company.

In the Supreme Court, business advocates—including the U.S. Chamber of Commerce, The Business Roundtable and the Retail Litigation Center—have urged the justices to uphold the lawfulness of arbitration agreements in employment contracts.

According to the report, conducted by Cornell professor Alexander J.S. Colvin, among non-union private-sector employers 53.9 percent have mandatory arbitration procedures and among companies with 1,000 or more employees, 65 percent have such procedures. Of the employers who require mandatory arbitration, 30 percent also include class action waivers, the report found.