The Government Accountability Office warns in a new study that the long-term stability of traditional, defined benefit pension plans "is now vulnerable to the broader trends of eroding retirement security."

Millions of Americans still rely on the plans, which offer lifetime retirement benefits based on an employee's years of service with a company. But the GAO study found more than 80 percent of the companies it surveyed had already modified their pension plans, usually because of market pressures, and that was before the financial crisis began last fall.