The nation's largest provider of security guards plans to discontinue its lowest-cost health plans and steer roughly 55,000 workers to new government-sponsored insurance exchanges for coverage next year, in the latest sign of the fraying ties between employment and health care.

The U.S. arm of Sweden's Securitas AB is among more than 1,200 employers that offer the kind of bare-bones health plans that must be phased out beginning Jan. 1 under the health-care law. Nearly four million people are enrolled in these so-called mini-med...