Scientists have shown it is possible to fake DNA evidence, potentially undermining the credibility of the key forensic technique.
Using equipment found in labs up and down the country, they obliterated all traces of DNA from a blood sample and added someone else's genetic material in its place.
The swap was so successful it fooled scientists who carry out DNA fingerprinting for U.S. courts.
Forensic evidence, often analysed in crime dramas such as CSI: Crime Scene Investigation (above), may not be the gold standard for such cases any more
The development raises the possibility of samples of blood or saliva being planted at crime scenes, leading to the innocent being wrongly convicted and the guilty going free.
Israeli researcher Dan Frumkin, who produced the bogus DNA, said: 'If you can fake blood, saliva or any other tissue, you can engineer a crime scene. Any biology undergraduate could perform this.'
Dr Frumkin's company, which has made a kit he claims can distinguish real DNA samples from fake ones, used two techniques to fabricate the evidence.
In the first, they extracted minute samples of genetic material from strands of hair and multiplied them many times over.
They then inserted this DNA into blood cells that had been purged of all genetic clues to their real owner.
The blood then contained the genetic fingerprint of the first person, the journal Forensic Science International: Genetics reports.
In theory, it could then be planted at the scene of a crime.
DNA material can be gathered from an empty wine glass or cigarette butt
Hair, chewing gum, cigarette butts and mugs and glasses could all provide an initial DNA sample.
The company has also developed a more complicated technique, which relies on knowledge of the DNA fingerprint - a 'bar code' of genetics from 20 set spots on a person's DNA.
The scientists built a 'library' containing hundreds of DNA snippets covering all the genetic codes that crop up at the set points scrutinised by police. To make a sample matching a particular fingerprint, they just dipped into their library for the right combination and mixed them in a test-tube.
The researchers believe eventually the technology will be used by criminals.
They warned: 'DNA evidence is key to the conviction or exoneration of suspects of various types of crime, from theft to rape and murder. However, the disturbing possibility that DNA evidence can be faked has been overlooked.
'DNA with any desired genetic profile can be easily synthesised using common and recently developed biological techniques, integrated into human tissues or applied to surfaces of objects, and then planted in crime scenes.'
But British experts said it was highly unlikely any criminal would go to such lengths. Dr Gill Tully of the Government-funded Forensic Science Service said: 'You would need a full molecular biology lab, thousands of pounds worth of equipment and a fully competent molecular biology scientist or technician.
'The vast majority of people who may be involved in criminality would not have access to these materials.'
Dr John Manlove, a forensic scientist and expert witness in court cases, said: 'Yes, it is scientifically possible but it is somebody going to an extreme.
'DNA is very important to a case but the investigations are not carried out on the basis of DNA alone.'
