Forensic evidence that has helped convict thousands of defendants for nearly a century is often the product of shoddy scientific practices that should be upgraded and standardized, according to accounts of a draft report by the nation’s pre-eminent scientific research group.

Robert L. Stinson, convicted of murder in 1984, was freed from a Wisconsin prison last month after tests found that bite-mark and DNA analysis did not match evidence from the crime scene.

The report by the National Academy of Sciences is to be released this month. People who have seen it say it is a sweeping critique of many forensic methods that the police and prosecutors rely on, including fingerprinting, firearms identification and analysis of bite marks, blood spatter, hair and handwriting.

The report says such analyses are often handled by poorly trained technicians who then exaggerate the accuracy of their methods in court. It concludes that Congress should create a federal agency to guarantee the independence of the field, which has been dominated by law enforcement agencies, say forensic professionals, scholars and scientists who have seen review copies of the study. Early reviewers said the report was still subject to change.

The result of a two-year review, the report follows a series of widely publicized crime laboratory failures, including the case of Brandon Mayfield, a lawyer from Portland, Ore., and Muslim convert who was wrongly arrested in the 2004 terrorist train bombing in Madrid that killed 191 people and wounded 2,000.

American examiners matched Mr. Mayfield’s fingerprint to those found at the scene, although Spanish authorities eventually convinced the Federal Bureau of Investigation that its fingerprint identification methods were faulty. Mr. Mayfield was released, and the federal government settled with him for $2 million.

In 2005, Congress asked the National Academy to assess the state of the forensic techniques used in court proceedings. The report’s findings are not binding, but they are expected to be highly influential.

“This is not a judicial ruling; it is not a law,” said Michael J. Saks, a psychology and law professor at Arizona State University who presented fundamental weaknesses in forensic evidence to the academy. “But it will be used by others who will make law or will argue cases.”

Legal experts expect that the report will give ammunition to defense lawyers seeking to discredit forensic procedures and expert witnesses in court. Lawyers could also use the findings in their attempts to overturn convictions based on spurious evidence. Judges are likely to use the findings to raise the bar for admissibility of certain types of forensic evidence and to rein in exaggerated expert testimony.

The report may also drive federal legislation if Congress adopts its recommendations. Senator Richard C. Shelby, Republican of Alabama, who has pushed for forensic reform, said, “My hope is that this report will provide an objective and unbiased perspective of the critical needs of our crime labs.”

Forensics, which developed within law enforcement institutions — and have been mythologized on television shows from “Quincy, M.E.” to “CSI: Miami” — suffers from a lack of independence, the report found.

The report’s most controversial recommendation is the establishment of a federal agency to finance research and training and promote universal standards in forensic science, a discipline that spans anthropology, biology, chemistry, physics, medicine and law. The report also calls for tougher regulation of crime laboratories.