States increasingly are imposing fees on poor criminal defendants who use public defenders even when they can't pay, causing some to go without attorneys, according to two reviews of the nation's largest state criminal justice systems.

A report out Monday by New York University School of Law's Brennan Center for Justice found that 13 of the 15 states with the largest prison populations imposed some charge, including application fees, for access to counsel.

"In practice, these fees often discourage individuals from exercising their constitutional right to an attorney, leading to wrongful convictions, over-incarceration and significant burdens on the operation of courts," the Brennan report concludes.

In Michigan, the report says, the National Legal Aid and Defender Association found the "threat" of having to pay the full cost of assigned counsel caused misdemeanor defendants to waive their right to attorneys 95% of the time.

Three states studied — Florida, North Carolina and Virginia — have no provisions for the courts to waive some of the fees if defendants can't pay. In Virginia, defendants may be charged up to $1,235 per count for some felonies, the report says.

A separate report of five state justice systems out Monday by the ACLU produced similar findings.

Both studies say the fees are a little-known source of revenue in the criminal justice system that are steadily rising.