WASHINGTON — In a move that could brighten retirement prospects for millions of Americans, the U.S. Supreme Court ruled employers have a duty to keep watch over 401(k) plans to guard against high management fees that can erode retirement savings.

A unanimous high court said Monday employers can be sued if they fail in their “continuing duty to monitor” mutual funds in 401(k) accounts for unnecessarily high fees, potentially shaking up the $5.8 trillion market for administering the plans.

The decision involving Edison International in Rosemead, Calif., surprised analysts both for its sweep and the agreement of the frequently fractured court. The ruling effectively shifts the burden in disputes over monitoring retirement plans from workers to the employers that administer them.

Employees long have been saddled with 401(k) plans that limit investment choices and provide no way to negotiate for better plans or lower fees. Workers are left largely to themselves to fund and oversee their retirement plans.

Management fees typically are buried deep in retirement documents and can go unnoticed by most workers. They are small — a percent or two — but they can do major damage to an investment portfolio over time.

A 1 percent fee on a $100,000 portfolio, for instance, would cost a retirement fund $30,000 more than a fee of 0.25 percent over two decades, assuming an annual 4 percent rate of return, according to the Securities and Exchange Commission.

Put another way, an employee with the lower-cost fund would have $30,000 more to rely on in retirement. The larger the nest egg, the wider the gap would be.

The Supreme Court decision is the second major setback this year for Wall Street and the rest of the financial services industry, which in recent decades has taken over managing an increasing share of the nation's retirement system. Though the court decision applies to employers, the industry will be pushed to lower fees.

Last month, the U.S. Department of Labor formally proposed a new rule that would require investment advisers to put clients' interests first across a broad swath of retirement-related transactions. That bitterly contested rule has big implications for the $6.5 trillion market for Individual Retirement Accounts, which differ from 401(k)s in that the personal accounts don't involve employers.

Teresa Ghilarducci, a labor economist and retirement scholar at New York's New School, said both the Labor Department proposal now out for public comment and the Supreme Court decision together represent a major shift in the retirement policy discussion.