Attorney General Eric H. Holder Jr. on Thursday reversed a controversial Bush administration policy under which numerous defendants have waived their right to DNA testing even though that right is guaranteed under federal law, the Justice Department said.

Holder issued a memo to the nation's federal prosecutors overturning the policy of seeking "DNA waivers,'' the department said in a statement.

The waivers have been in widespread use in federal cases for about five years and run counter to the national movement toward allowing prisoners to seek post-conviction DNA testing to prove their innocence. More than 260 wrongly convicted people have been exonerated by such tests, though virtually all have been state prisoners.

The waivers are filed only in guilty pleas and bar defendants from ever requesting DNA testing, even if evidence emerges that could exonerate them. Statistics show that innocent people sometimes plead guilty, often for a reduced sentence. One quarter of the 261 people who have been exonerated by DNA testing had falsely confessed to crimes they didn't commit, and 19 of them pleaded guilty, according to the New York-based Innocence Project.

Holder last year ordered a review of the little-known policy, under which the Bush Justice Department had told prosecutors to seek DNA waivers whenever possible. The review came after inquiries about DNA waivers by The Washington Post.

The waivers have been part of the standard plea agreement filed by some of the nation's most prominent U.S. attorneys, including those in the District, Alexandria and Manhattan. Defense lawyers say their clients are essentially forced to sign the waivers or lose the benefits of a plea agreement, such as a lighter sentence.

Holder's memo said the DNA waiver policy is inconsistently applied and is "too rigid to accommodate the facts presented by individual cases.'' As of last year, at least 19 U.S. attorneys' offices used the waivers for some or all plea agreements, while 24 U.S. attorneys did not use them. It could not be determined how many inmates have been affected by the policy, because the remaining U.S. attorneys' offices did not respond to inquiries or declined to comment.

Now, Holder's memo says prosecutors can only seek the waivers in "exceptional circumstances" andwith a supervisor's approval.

The new policy on DNA waivers came with another Holder memo, also issued Thursday, that clarified Justice Department guidelines for collecting DNA samples from federal prisoners. The memo noted that regular collection of such samples must be a priority.

"DNA evidence is one of the most powerful tools available to the criminal justice system, and these new steps will ensure the department can use DNA to the greatest extent possible to solve crimes and ensure the guilty are convicted," said Holder, a former U.S. attorney in the District who has called for expanded DNA testing in federal courts.

"Improving both the collection and the use of DNA evidence will help law enforcement and prosecutors keep communities safe,'' he said.

Sen. Patrick J. Leahy (D-Vt.) on Thursday commended Holder for "reversing the Bush administration policy of demanding that defendants waive their right to DNA testing as a requirement of certain plea negotiations.''