The Federal Bureau of Investigation (FBI) has admitted decades of forensic hair analysis used as evidence in criminal trials was wrong.

An investigation looking at hundreds of court cases has revealed nearly every examiner in an FBI hair forensic unit gave flawed testimony.

In the great majority of cases the evidence favoured the prosecution.

In some cases the defendants convicted by the flawed analysis are no longer around to appeal - because they were executed.

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The Innocence Project has long argued thousands of American criminal cases need to be reviewed because they relied on hair analysis.

It has been helping the FBI review hundreds of convictions.

Justin Brooks, director of the California Innocence Project, said "the results are absolutely shocking".

In the era before courts started relying heavily on DNA, in the 1970s, 1980s and 1990s, experts from the FBI Laboratory's microscopic hair unit were regulars in American courts.

Prosecutors used them to give their opinion on whether a defendant's hair matched hair found at a crime scene.

"The FBI examiners overstated the science in their testimony and in the United States it is incredibly powerful when experts testify to jurors," Mr Brooks said.

The Washington Post got hold of the results of the review, and they show that in 95 per cent of cases, the experts gave bogus forensic hair evidence against a defendant.

Mr Brooks said false testimonies in 257 cases most often favoured the prosecutors.

"This is what you have to do when science moves on and you learn we were wrong about things in the past," he said.

"The problem in these cases is some people have been executed who had this kind of testimony presented in their cases."

DNA evidence more reliable than hair analysis: expert

Dr Claire Ferguson, a lecturer in Forensic Criminology at the Queensland University of Technology, said DNA evidence is now more commonly used in criminal cases.

She said there had long been concerns about forensic hair analysis.

"DNA from hair is a much more reliable methodology for determining whether or not there's consistencies."

The revelations are not just a question of 20-20 hindsight.

As far back as 1994, former FBI forensic scientist Dr Frederic Whitehurst raised concerns about flawed forensic procedures within the FBI.

His lawyer, David Colapinto from the National Whistleblowers Centre in Washington DC, said Dr Whitehurst had been vindicated.

"He was concerned about a number of systemic problems in the lab ... [Dr Whitehurst] was severely criticised for pointing out that this was a systemic problem," Mr Colapinto said.

"I think the Justice Department owes him an apology."

The FBI said it is now conducting an independent investigation to find out why false evidence was given for decades.

There are still more than 1,000 cases to review.