A mother who allowed her partner to violently murder her “blameless” son days after his second birthday has been jailed for eight years.

Sarah O’Brien, 33, was found guilty of causing or allowing the death of Keigan O’Brien and child cruelty after a trial at Sheffield Crown Court lasting nearly five weeks.

Martin Currie, her partner of six months, was jailed for a minimum of 22 years for murdering the Doncaster toddler, who was found to have sustained numerous injuries in the weeks leading up to the “catastrophic” head injury that caused his death.

Sentencing Currie, Mrs Justice Eady said: “I must sentence you for the violent murder of a blameless two-year-old left in your care.

“These offences involved the grossest abuse of a position of trust.

“You placed yourself in the role of Keigan’s father, your duty was to protect him, instead you abused, injured and killed him.

“You inflicted appalling physical suffering on Keigan before he died.”

To O’Brien, the judge said: “The jury found that you were aware, or ought to have been aware, of the risk of serious physical harm posed to Keigan by Martin Currie.

“At the very least, you failed to protect Keigan, leaving him with Martin Currie to be subjected to the very significant force used in the incident that led to his death.”

Mrs Justice Eady said heroin addict Currie was “apparently triggered by a fit of temper” at being interrupted by Keigan as he “lay in bed looking at porn” on his phone while O’Brien was out of the home in Bosworth Road, Adwick-le-Street, on the morning of January 8.

She said he “violently yanked” Keigan’s arm, fracturing his wrist, shook him and threw him down, hitting his head on a hard surface, before leaving him to “bleed on to his pillow” and ignore him while he returned to browsing gambling and porn sites.

She said: “It’s chilling to think that the gaps in your internet use provide the best evidence as to the time Keigan was fatally assaulted.”

The judge said Currie, 36, went on to search the internet for terms including “irregular breathing”, “unconscious” and “gurgling” and, “in a particularly pathetic sign of your self-absorption” made contact with his drug dealer.

But he did not call an ambulance until around two hours later, after O’Brien had returned home, when he “engaged in a cruel charade, purporting to try to carry out CPR on the child you had viciously assaulted and left seriously injured”, the judge said.

Keigan died from a bleed on the brain in hospital the next day, just three days after his birthday.

Mrs Justice Eady said it was discovered the toddler had been subjected to “extreme acts of violent abuse” and was found to have a fractured arm, sustained around the same time as the head injury, a fractured spine, fractured ribs and previous bleeding on the brain, all caused in the weeks and months before he died.

Currie and O’Brien lied to the emergency services and the police, claiming to have found Keigan not breathing in bed and did not explain how he came by his injuries.

But the judge described Currie’s evidence in court, which included him claiming he heard the boy’s mother assaulting him previously but had not witnessed it, as a “pack of lies” and said the jury was sure he had inflicted the fatal injuries.

She said to O’Brien: “It may be that, in your distress, you managed to persuade yourself that you could not be to blame, but you failed to say what you must have known: that Keigan must have been fatally injured by the man you had left him with that morning.”

O’Brien, 33, embarked on the relationship with Currie six months before Keigan’s death, despite warnings not to go out with him, and witnesses noticed a change in her attitude and behaviour towards her son.

Mrs Justice Eady said O’Brien decided to put her relationship above the interests of her child and said the pair adopted “cruel and dehumanising names” for Keigan, referring to him as “wankstain” and “shithead”.

In October, O’Brien did not seek medical help when her son suffered two black eyes while in the care of Currie, telling her friend the toddler had fallen down the stairs.

O’Brien explained to the friend she did not take him to hospital because “they will start asking questions”.

Investigations after the murder found blood spatters from a previous occasion on the wall by Keigan’s high chair and on Currie’s clothes.

Jurors returned their verdicts on Tuesday, after deliberating for nearly 13 hours, finding Currie guilty of murder and child cruelty.

O’Brien was cleared of murder and manslaughter but was convicted of causing or allowing the death of a child and child cruelty.