Madeleine: 'Fatal flaw' over test that found DNA in parents' hire car

The alleged "smoking gun" evidence used to implicate Madeleine's parents in her killing could be fatally flawed, experts said last night.

British forensic scientists identified Madeleine's DNA in a car hired by her parents five weeks after her disappearance and this unexplained evidence has been used to imply that the little girl or her body must have been moved by the McCanns weeks after her alleged abduction.

But research papers written by the scientists involved in the case suggest that there could easily be an innocent explanation for the minute traces found in the car.

The Mail on Sunday understands that the DNA particles in the vehicle were uncovered using an advanced technique pioneered by the UK Forensic Science Services (FSS) laboratories in Birmingham.

The so-called "low copy number DNA" evidence allows for minuscule biological samples to be analysed to detect very low quantities of DNA - even as little as a single cell.

This sort of evidence has proved crucial in rape and murder cases, particularly where the suspect has no other connection to his or her victim.

But a scientific paper on the technique written by six leading FSS scientists reveals that while the tiny DNA samples can be positively identified, there is no way of knowing how the particles are deposited.

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The scientists warn that where this DNA is being used in a trial, "specific caveats are written into court statements" and they point out that it is not possible to make conclusions about how the tiny traces of DNA are deposited.

They also warn that there is a danger that the DNA can be moved from one individual to another and then on to an object.

Experiments showed this transfer could take place weeks or months and, in the case of one item tested, a glove, two years later.

For the McCanns, this leaves open the possibility that Madeleine's DNA was transferred by them or by an item impregnated with her cells, like an item of clothing or her cuddly toy bunny, which Kate McCann has carried constantly since her daughter's disappearance.

The revelation once again puts the spotlight on dangers of overreliance on forensic evidence which has led to a string of miscarriages of justice in the UK and disquiet about the safety of convictions in other high-profile cases.

Recently there have been questions about the guilt of Barry George, serving life for killing the TV presenter Jill Dando, because of doubts over the forensic evidence in the case.

He has been given leave to appeal after a review of the case said "too much significance" was placed on a single, tiny speck of shotgun residue - invisible to the naked eye - found in George's coat, and that there is a "real possibility" his conviction could be quashed.

The House of Commons called for more scepticism about scientific evidence in court after paediatrician Sir Roy Meadows was discredited over evidence he gave in a number of cases involving baby deaths.

His "Meadow's law" on cot deaths "that one in a family is a tragedy, two is suspicious and three is murder" formed part of the case against Angela Cannings, who was wrongly convicted of killing her two sons.

But perhaps the most high-profile case where forensic science has been called into question is the conviction of six men for the Birmingham pub bombings in 1974.

They spent 16 years in prison after being wrongly convicted of the murder of 21 people but were released after it was discovered that the forensic evidence used to convict them, which suggested they had handled explosives, was unreliable and gave a false positive result when people touched household items such as playing cards.