The number of rapists given a caution and freed instead of facing jail has more than doubled in the past decade in England and Wales, figures show.
The number of rapes reported to the police is rising year on year
In 2004, 40 offenders were cautioned for rape - compared with 19 in 1994.
The Home Office said cautions were used only in the most exceptional cases, but campaign group Rape Crisis said it was "shocked" by the statistics.
Most of those cautioned are thought to be either under-18s or older people who committed rape a long time ago.
The number of rapes reported to the police is rising year on year.
But the proportion resulting in a conviction is falling - from about one in three in 1977, to one in 20 in 2004.
The Home Office said it was committed to bringing more rapists to justice.
A Home Office spokesman said: "Rape is an appalling crime, it devastates the lives of victims and their families, however rape will always be a difficult offence to prosecute."
The Sexual Offences Act 2003 created a clearer definition of consent and set out how a defendant in a rape case needed to show grounds to believe they had consent.
Criminal barrister Kirsty Brimelow, who has defended men in rape cases, said about half of those cautioned in 2004 were under 18 years old.
She said some of the others were likely to be historic rape allegations, where the crime was committed decades earlier.
An admission of guilt is needed for a caution.
Admitting guilt means being put on the sex offender register.
The caution will show up on a search of criminal records.
The CPS decides how to handle rape cases.
The CPS says cautions are only used in exceptional cases.
Rape carries a maximum life sentence.
"Sometimes in those cases the victim just wants an admission from the defendant, but doesn't want a trial so that would result in a caution," she said.
Claire Ward from the Crown Prosecution Service said people were now encouraged to make historic allegations.
She also pointed out the numbers of rapists receiving cautions had remained fairly constant since 2000.
The CPS said cautions were given only in "extreme circumstances" where the accused admitted the offence.
One example from last year was a pensioner who accepted a caution for raping his sister when they were children, some 50 years earlier.
In another case from 2004, a 13-year-old boy was given the equivalent of a caution for raping a young child.
Rape Crisis chairwoman Nicole Westmarland said she was shocked that so many cautions had been given.
"It is completely unacceptable that rapists are able to continue living their day-to-day lives or even be free to rape again.
"Rape is a crime that has a serious impact on its victims for years or even decades."
Liberal Democrat Home Affairs spokesman Nick Clegg told BBC Radio 4's Today programme that there were circumstances where a caution for rape would be appropriate.
"If the victims themselves don't want to go through a whole trial in court, if it involves very young kids, or if it's an offence that maybe took place decades ago," he said.
In 2001 there were an estimated 80,000 incidents of rape or attempted rape against women aged between 16 and 59 in England and Wales.