Supreme Court Justice Clarence Thomas failed to report his wife's income from a conservative think tank on financial disclosure forms for at least five years, the watchdog group Common Cause said Friday.

Between 2003 and 2007, Virginia Thomas, a longtime conservative activist, earned $686,589 from the Heritage Foundation, according to a Common Cause review of the foundation's IRS records. Thomas failed to note the income in his Supreme Court financial disclosure forms for those years, instead checking a box labeled "none" where "spousal noninvestment income" would be disclosed.

In his 2009 disclosure, Justice Thomas also reported spousal income as "none." Common Cause contends that Liberty Central paid Virginia Thomas an unknown salary that year.

Federal judges are bound by law to disclose the source of spousal income, according to Stephen Gillers, a professor at NYU School of Law. Thomas' omission — which could be interpreted as a violation of that law — could lead to some form of penalty, Gillers said.

"It wasn't a miscalculation; he simply omitted his wife's source of income for six years, which is a rather dramatic omission," Gillers said. "It could not have been an oversight."