Adeyoola was found guilty of murdering 84-year Anne Mendel, who was found by her husband in March last year lying in a pool of blood at the bottom of the staircase in her home. She had been stabbed 14 times. The DNA of the 18-year-old, who had formerly been a neighbour, was found on Mrs Mendel's hand, a fact Adeyoola explained away by saying she had, by chance, helped the elderly woman across the road earlier that day.

Adeyoola pleaded not guilty, but the prosecution argued successfully that she had carried out the murder herself, possibly as a "dry run" for the slaughter of a wealthy woman who would make a more lucrative "job". It was a killing the teenager had meticulously planned in a murder manual written a few months earlier when she was in juvenile custody.

Staff at the Bullwood Hall juvenile offenders' institution, where Adeyoola was serving a sentence for shoplifting, found the blueprint in October 2004, during an inspection of her cell. On being confronted the teenager claimed the whole thing was fiction. But within the space of a few months the fiction turned into reality.

The handwritten notes, entitled "Prison and After: Making life count", began innocently, with plans to "lose two stones from 11 stone eight to nine and a half stone", and "locate two-bed apartment". Adeyoola, herself the daughter of a millionaire property developer, went on to outline how she would obtain bank accounts, "make £2,000" to furnish her apartment, buy a Mercedes and clothes and DVDs. Despite having been privately educated Adeyoola left school with no qualifications, and wanted to obtain fake GCSE and A-Level qualifications over the internet, and mock up a CV stating she had attended the Chelmsford School for Girls.

But "the main changes for [a] happy future" depended on Adeyoola getting "a minimum of £3m", according to the journal. For this she would need to rob and murder an elderly woman, and frame the woman's husband for the killing. Such a feat would require equipment: petrol canisters, plastic bags, cling film, semi-automatic guns, Taser stun guns and a bullet-proof vest were all duly listed. So were various disguises - wigs, dark glasses, and bizarrely, a "full body fat suit, approximately 18 stone woman".

She sketched out her ideal victim: "She must be wealthy, quite elderly and defenceless. Find a candidate and watch her. Their routine must be closely observed. Follow her. She will probably be married. See if they get any visitors. Create a survey questionnaire and visit them disguised as an A-level student. Tell them it's field work. Ask them: 'Do you have guns to protect yourself?'"

The attack was imagined in emotionless detail: lie in wait for the victim, creep up on her, and cover her mouth with a gloved hand. Once in the house, threaten the victim with a knife, extract bank account details and the code to her safe, get her to write a fake 'I'm leaving' note, and then despatch her.

"With your butcher's knife remove her head. Wrap it in film to contain bleeding, detach limbs one by one," she wrote. "When you have completed the task, put head, body pieces in black bag."

The "job" was to be done by February 2005.

Throughout her trial Adeyoola insisted the document was a work of fiction, notes for a crime thriller she was writing in the style of a James Patterson book. Following the discovery of the papers in her cell she was interviewed by a senior staff member and psychologist, whom she told: "I want them back. It's a story."

In court she dismissed the manual as "the scribblings of a 16-year-old girl". She said she was fascinated by writers such as Martina Cole and James Patterson, and wanted to emulate them.

"I had always considered writing a book. I'd like to be an author. When I was in Bullwood Hall I thought it would be a good place to start," she told the court.

In her evidence the teenager also told how she had set up house with another girl in Belsize Park, north London, and worked as an escort to pay the rent. "All we had to do was go out on a date and we could make £500 a night. We could make up to £5,000 a week," she said. "It was safe because we shadowed each other and it was easy."

A police source described finding a cache of sex toys when Adeyoola's flat was raided after she was arrested.

"There were several vibrators, nipple clamps, lesbian pornographic magazines, edible underwear made of candy, revealing outfits, handcuffs - all sorts of equipment," the source said. Police also discovered piles of stolen designer clothes and a book entitled Great Crimes.

Adeyoola also told the court she had begun stealing from shops at age 15, something she described as a "skill". She earned a string of convictions for theft, which culminated in her serving three months in a young offenders' institution, where she hatched her murder plans.

The young woman was also charged with two counts of peverting the course of justice, after she was taped setting up a bogus alibi for the murder with a 16-year-old accomplice. She and the 16-year-old, who cannot be named for legal reasons, were convicted on one of those counts.

Speaking after the verdict, Detective Inspector Steve Morris of the Metropolitan police described the case as "a wicked, premeditated murder committed by a callous, devious young woman and her cold, calculated use of extreme violence beggars belief".

Her father, Bola Adeyoola, who runs a successful property management company, said: "What she did was evil."

Presiding Judge Richard Hone QC suggested she may have even committed the murder to amass "material" for her future literary endeavours.

Adeyoola was remanded in custody to await sentencing next month.

A life of privilege

Kemi Adeyoola is the daughter of Bola Adeyoola, a property manager and former boxer who is worth an estimated £10m. He said yesterday that his daughter was a spoilt, cunning girl who had been obsessed with money from an early age. He lost contact with her when she was 11. After that, he said, he passed money and messages to her through her grandparents.

Following her release from Bullwood Hall prison he set her up in a flat but they fell out when he advised her to "go straight". According to Mr Adeyoola, she responded by vandalising his office. Despite the discovery of the "blueprint for murder", psychologists at Bullwood Hall had held high hopes for Adeyoola on her release, describing her as an articulate, "intelligent and sophisticated" girl.