WASHINGTON (January 9, 2013)—The nation's tax law is so thick and complicated that businesses and individuals spend more than 6 billion hours a year complying with filing requirements, which is the equivalent of 3 million people working full-time, year-round, a government watchdog group says.

As a result, about 90 percent of filers will either pay a tax preparer or use a computer software service to help file their federal tax returns this spring.

National Taxpayer Advocate Nina E. Olson ranked the complexity as the most serious tax problem facing taxpayers and the IRS in her annual report to Congress on Wednesday.

Momentum is building in Congress to overhaul the tax code for the first time since 1986, but Washington's divided government has yet to show it can successfully tackle such an issue.