The boy held his head in his hands and sobbed as he was convicted of raping a five-year-old girl in a cemetery in Leeds.
A second 12-year-old boy, the girl's uncle, has also admitted indecently assaulting his niece during the attack.
Both boys, now aged 13, will be placed on the Sexual Offenders Register and will have to report their movements to the police. They were both released on bail yesterday and will be sentenced after reports have been prepared.
The minimum age a person can be convicted of rape was lowered in 1993 from 14 to 10. The Sex Offences Act 1993 abolished the presumption that boys aged 10 to 13 were not considered capable of committing rape. The change in the law followed an outcry over a number of sexual assaults by children.
In what may be the only other case involving such a young offender, a Somali refugee was given five years' detention in January last year after being found guilty of raping a tourist in London when he was 12.
In the latest trial, the jury at Leeds Crown Court heard that the girl was raped as she played with the two boys next to Harehills cemetery in Leeds on a Sunday afternoon last September.
The court heard that when the girl, who gave evidence via a video link, returned home, she told her grandmother that the boy got on top of her and told her he was going to show her "what daddies do to mummies".
Nicholas Campbell, for the prosecution, said the 12-year-old raped the girl the first time and then threatened her uncle that he would burn his clothes on a fire they had started if he did not have sex with the girl as well.
After the uncle had sex with her, the first boy raped her a second time.
In a videotaped interview with police, the girl described how she tried to fight the 12-year-old off. "I was saying, `Get off me, get off me', and I was kicking my legs at him. But he wouldn't get off," she said. She said that the 12-year-old hurt her and laughed as he raped her, and she thought they were going to kill her.
After her grandmother called police, the girl was taken to hospital and examined by a doctor, who found evidence that she had been sexually assaulted.
The girl's uncle, who had been living with the girl and who had given evidence for the prosecution, admitted he got on top of the girl when he was threatened but did not have full sex.
Neither of the boys can be named for legal reasons.
After the verdict, during which the boy's mother comforted her son, Judge Arthur Myerson QC said: "It was a very difficult and somewhat emotional case. It's a verdict I can well understand."
Last October a 10-year-old boy was charged at Nottingham Crown Court with the rape of a 12-year-old boy, but he was convicted of indecent assault.
In February this year, a 12-year-old in Sunderland became the youngest person to be placed on the Sex Offenders Register. He admitted indecently assaulting two children aged six and seven.
Two boys aged 10 were to be charged with rape, but were cleared on the orders of an Old Bailey judge in February.
In 1996 there were two conviction of rape carried out by boys aged between 10 and 14.Reuse content