As Congress nears votes on legislation that would overhaul the health care system, many small businesses say they are facing the steepest rise in insurance premiums they have seen in recent years.

Insurance brokers and benefits consultants say their small business clients are seeing premiums go up an average of about 15 percent for the coming year  double the rate of last year’s increases. That would mean an annual premium that was $4,500 per employee in 2008 and $4,800 this year would rise to $5,500 in 2010.

The higher premiums at least partly reflect the inexorable rise of medical costs, which is forcing Medicare to raise premiums, too. Health insurance bills are also rising for big employers, but because they have more negotiating clout, their increases are generally not as steep.

Higher medical costs aside, some experts say they think the insurance industry, under pressure from Wall Street, is raising premiums to get ahead of any legislative changes that might reduce their profits.