A 13-year-old boy was in secure custody last night after a court heard how he killed his baby brother by stabbing him 17 times and cutting off his left hand.

Bristol Crown Court was told that the boy, who suffers from a serious psychiatric illness, had an appalling and wretched upbringing leading up to the killing on January 19 last year, when he was aged 12.

Yesterday, his guilty plea to manslaughter was accepted and the court adjourned the case for the boy to be sentenced later.

The court heard how police officers found the six-month old baby with horrific injuries in a cot at his home, in the Withywood area of the city.

They had been alerted when the boy walked into a police station near his home and told them he had stabbed his baby half-brother before giving them a 5in kitchen knife.

Mr Roderick Denyer, prosecuting, said that when interviewing officers asked why he had done it, the boy replied: "I don't know."

The post mortem examination revealed that the baby had 17 stab wounds, including one which severed his spinal cord. His left hand had been severed at its wrist.

Mr Denyer said the first his mother knew of the shocking death of her baby was when the police arrived at her home, as she had been "dozing" downstairs.

She told officers she had argued with the boy, who had gone upstairs to feed his half-brother. She said he returned a few minutes later, put his clothes in a wash bin and walked out of the house.

Mr Denyer was told of the boy's "wretched" upbringing and how he suffered from serious psychiatric disorders, including the autism-type illness Asperger Syndrome.

Lady Helena Kennedy QC, defending, said: "For any of us, it was clear that this was not a cold act of malevolence."

But she said the boy was brought up "in circumstances of serious deprivation".