Kemi Adeyoola, 18, was found guilty at the Old Bailey of murdering Anne Mendel, 84, after she outlined the crime in an 18-page “murder manual” written while she was in a young offender’s institute.

Not long after serving three months of a six-month sentence for shoplifting, the teenager entered the home of Mrs Mendel, her former neighbour, and stabbed her 14 times.

Adeyoola later told detectives that the manual, which contained details of how she would kill and rob a “rich, elderly and defenceless” victim, was a draft for a crime novel she was writing.

Mrs Mendel was found by her husband in the hallway of their home in Golders Green, northwest London, in March last year.

Adeyoola had chosen the pensioner as a soft target on which to practise before finding another victim in a scheme to get rich, prosecutors said.

But yesterday, the judge suggested that the teenager had killed in order to gain first-hand experience for her writings. Judge Richard Hone, QC, said: “There is a worrying feature that this may have been done as an experiment for a future literary endeavour.”

Described by prosecutors as cunning, highly manipulative and devious, the former public schoolgirl had apparently planned the murder while at the Bullwood Hall Young Offender Institution, in Essex.

The jury was asked to compare Adeyoola’s writings, which were found in her cell, with those of her favourite crime writer, James Patterson, the author of Along Came a Spider, whom she claimed to emulate.

Her neatly written notes, entitled “Prison and After. Making Life Count from 20 August to 20 November 2004”, listed Adeyoola’s ambitions, including her aim to make £3 million by robbing houses.

The manual also contained plans to obtain guns, knives, cars, a bulletproof vest, a disguise and drugs with which to tranquillise and kill a wealthy woman, the court was told. “She must be wealthy, quite elderly and defenceless,” Adeyoola wrote of her potential victim.

After robbing her, she planned to cut the woman up and dispose of the body in bin liners or in a burning car.

Adeyoola told the court that the work was a draft for a crime thriller.

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