The 41-year-old victim was also raped and sexually assaulted during the 12-hour ordeal at her own home, Bradford Crown Court was told.

The "systematic and deliberate torture" all took place on her birthday.

Charles Collins, 45, and Kerry Farebrother, 21, both of Lustre Street, Keighley, pleaded guilty to wounding, sexual assault and causing actual bodily harm.

Farebrother, who also admitted rape, was given an indeterminate jail sentence for public protection after Judge Jonathan Durham Hall QC branded him "a very evil man."

He told him he would have to serve at least six years behind bars before being considered for parole, but said it was most unlikely that he would be released "for many, many years".

Collins was sentenced to nine years imprisonment.

Judge Durham Hall said the offences amounted to "the most appalling, degrading, humiliating and sadistic attack" on a vulnerable woman in her own home.

It had been "over a long period, without mercy, for thrills and for the joy of inflicting such humiliation, pain and distress on another human being."

Mark McKone, prosecuting, said the woman, who cannot be identified for legal reasons, met Collins in a pub in December last year and after he claimed to be homeless she allowed him to stay infrequently at her home.

In February this year he introduced her to Farebrother, claiming he was his son even though they are not related. Farebrother then started a relationship with the woman.

One evening in March the pair fell asleep after having sex. In the early hours of the following day Collins came into the bedroom, laughing and saying: "Let's play noughts and crosses."

He had a knife and while Farebrother held the naked woman's shoulders down he began cutting into her back. He then handed the knife to Farebrother.

"She was screaming and asking them to stop," said Mr McKone.

Later that morning, when she commented on their behaviour, Collins told her: "It was just for fun."

Farebrother went out to buy alcohol and some Ecstasy tablets. After he returned, he subjected the woman to a violent and sexual attack, and Collins joined in.

Farebrother punched her in the face and she fell to the ground. He put his hands round her throat and she was dragged along the floor. Collins shouted: "Go on son, kill her."

She was punched again and ordered to get on her knees. Farebrother then performed a sexual act on the woman who later described the pain as "excruciating."

Eventually, a neighbour heard her screams and called the police, who broke down the door and arrested the defendants.

Nichola Peers, for Farebrother, said he had had a troubled childhood and had been bullied at school.

Stephen Wood, for Collins, said his client suffered from a dependant personality disorder which rendered him unusually compliant and had a long history of drug and alcohol abuse. Farebrother became his registered carer.

"He came to rely on Farebrother utterly and that is how he came to be embroiled in these offences," added Mr Wood. "He was unable, through his condition, to take any practical steps to stop what was going on. He was terrified violence might be meted out to him."