Five people were cautioned for rape in the last year, a Freedom of Information request has revealed.
According to figures obtained by London Assembly member Tony Arbour, the Metropolitan Police issued 28,998 cautions in the period, a quarter of all solved crimes in the capital.
Among the cautions issued, 5,843 were given for violent attacks, 165 were handed out for sexual offences and five were for rape.
Cautions are issued by the police rather the Criminal Prosecution Service and mean people who commit crimes do not have to go to court.
Currently the Ministry of Justice is reviewing guidelines for issuing police cautions as part of a wider crackdown.
In November, a Sunday Mirror investigation revealed more than 1,800 sex offenders were given the slap on the wrist rather than being taken to court last year.
As many as 30 rapists nationwide – 11 of those with victims under 13 were given cautions, reprimands or final warnings.
Those who sexually assaulted children and made child porn also received cautions, reported The Sunday Mirror.
Policing Minister Damien Green said the government's goal was to see the use of cautions restricted to ensure serious criminals would never escape punishment.
Javed Khan, chief executive of the independent charity Victim Support, said: "Victims across London tell us they want the response to fit the crime and for that response to make sure the offender doesn't do it again.
"Although it is ultimately up to the police to decide on when to give a caution, victims must have decisions explained to them in order to retain their confidence in the police and other criminal justice agencies.
"The police need to be clear on when it is appropriate to give a caution - for example, this is not likely to be right for most violent and sexual offences."
Mr Arbour, a Conservative representing South West London on the Assembly, said: "The whole point of the British legal system is that justice should be seen to be done.
"However, a quarter of all solved crimes in London escape punishment.
"The 29,000 cautions and youth warnings given out in private during the last year demonstrate that criminals are getting away with increasingly serious crimes.
"It is unfair on victims, and the fact that a law-abiding resident will receive the same or even worse punishment for minor traffic offences as drug traffickers, robbers and rapists is simply outrageous."
Police handed out 205,700 cautions nationwide in the 12 months ending in September 2012, according to the latest figures from the Ministry of Justice.
It represented a 12% decrease compared with the previous 12 months, and a 44% fall from the 12 months to September 2007 when the use of cautions peaked at 367,300.