Failings by Devon and Cornwall Police meant historic rape cases went two decades without being solved, the police watchdog has said.
The Independent Police Complaints Commission said Devon and Cornwall Police "missed opportunities" to bring a conviction after it failed to upgrade DNA samples to a national database.
The review said a DNA sample from a 16-year-old girl who was raped in Plymouth in 1989 had not been loaded into the National DNA Database when it came into existence in 1995.
It meant that the man responsible, Shaun Harrison, did not get linked with the attack when a DNA sample was taken from him following his arrest for drink-driving in 2000.
It was only in August 2010 when the Forensic Science Service pointed out that the crime stain from the historic rape had not been uploaded that Harrison was eventually in the frame.
He pleaded guilty at court and was sentenced to eight years in custody for the 1989 rape, and a further four years for a rape he committed in 1994.
IPCC deputy chair Deborah Glass said: "Scientific advances have made it possible for historic crimes to be solved through the use of DNA analysis. However, that process is not automatic because of the cost involved. Our investigation found that Devon and Cornwall missed opportunities over the years to review this case.
"This meant that Harrison's and others' DNA samples were not upgraded in line with forensic advancements.
"The force has accepted all of the IPCC recommendations. A number of perpetrators of serious sexual assault crimes have since been brought to justice.
"The key issue identified by the IPCC is the way that police forces have to specifically request that DNA samples from historic cases be upgraded in line with forensic advancements."
In its report, the IPCC said Devon and Cornwall Police "did not appear to have... a central file store in existence until 1998 with their computerised crime recording system being introduced in 1992."
It added: "Therefore it is somewhat unsurprising that historic cases may remain undetected in storage boxes many years after their occurrence with no-one seemingly taking responsibility for its retention and case progression.
"Forensic science has long made a vital contribution to criminal investigations. In order for this contribution to be robust and thorough, police forces need to utilise their capabilities in a manner that is properly managed and organised if they are to make an efficient and effective impact on the detection and prevention of crime." There followed a complete review into unsolved sexual abuse cases with retained forensic material, including three cases from 1989, 1990 and 1993. Positive matches were obtained from the NDNAD in all three cases which resulted in investigations being commenced and led by the force's Major Incident Teams.
Two men were subsequently jailed following fresh investigations, the IPCC said.
Alexander Shepherd was jailed for six years and four months, after a 27-year-old woman was violently assaulted and raped in Plymouth in 1990, while William Poad was given a six-year sentence for an indecent assault on an 11-year-old girl in Penzance, Cornwall, in 1993, the IPCC said.
The third case, involving the rape of a 15-year-old girl in St Mawes in Cornwall in 1989 "did not proceed further", the IPCC concluded.
Devon and Cornwall Police issued a statement yesterday in which they welcomed and "fully accepted" the report.
The force added: "We have already adopted much of the learning contained within the IPCC report.
"This investigation has highlighted a potential national issue around how historic DNA should be treated. However, DNA technology has progressed significantly since 1989 and the way police investigate such offences is now very different.
"Victims of crime should be reassured that we will always seek to bring offenders to justice so that whenever possible they can get the outcome they rightly deserve."
