A mother and her partner have both been jailed for life with a minimum term of 39 years for murdering her son during a campaign of “horrific and prolonged torture”.

Sebastian Kalinowski died of an infection caused by “untreated complications of multiple rib fractures”, after weeks of what prosecutors described as “cruel assaults and abuse” at the hands of his mother Agnieszka Kalinowska, 35, and her long-term partner Andrzej Latoszewski, 38.

A jury at Leeds Crown Court heard the 15-year-old, who had been in the UK for less than a year after moving from Poland to live with his mother, was beaten with a bed slat, whipped with an extension cable and stabbed with a needle at the couple’s home in Huddersfield, West Yorkshire.

Sebastian’s father Jacek Kalinowski watched proceedings by video link from Poland.

Sentencing Kalinowska and Latoszewski, Mrs Justice Lambert told them: “You, Latoszewski, are a bully. Like all bullies, you sought to intimidate and frighten Sebastian, who was weaker than you.

“In Sebastian you found an easy prey.

“There were times when viewing the footage I was sure you had lost awareness of the fact that you were hitting a human being and not a punching bag.

“As for you, Kalinowska, the explanation is simple – you just didn’t care about Sebastian. You were only interested in yourself.”

The judge said that between the start of the school holidays in July and Sebastian’s death in August, there was “scarcely a day that went by that Sebastian was not subjected to some form of physical abuse”.

She told the defendants: “Sebastian was a young teenage boy who spoke little if any English, had neither friends nor allies in this country and was wholly dependent on you both for his wellbeing.”

Kalinowska sobbed as she was led out of the dock, while Latoszewski showed little emotion but had his head in his hands for most of the hearing.

Most of the trial jurors returned to court for the sentencing, while sobs could be heard from the public gallery as the judge described some of the abuse.

In a victim impact statement read to the court, Mr Kalinowski called Kalinowska “the greatest evil that walks this Earth”.

He said: “In a cruel and heartless way you have taken the life of my son. You turned every day of Sebastian’s life into a nightmare.

“Why did you hate him to that degree when you allowed that psychopath to treat Sebastian like that, and then you readily participated in the whole thing.

“You inflicted so much pain and humiliation on him. You made my son a punchbag for yourself, you committed a terrifying act and you were merciless.

“I made many mistakes as a parent. Perhaps I was not able to be a perfect father, but I loved Sebastian. However I could not protect him from evil, evil which was inflicted on him by his own mother.”

In mitigation for Latoszewski, Geraldine Kelly said: “However unreasonable it seems, and distorted a way of thinking, there is evidence to suggest what motivated this appalling conduct was them seeking to discipline (Sebastian).”

Mrs Justice Lambert was broadcast handing down her sentence – the third time cameras have been allowed into an English criminal crown court to record a sentencing.

The trial in June heard Sebastian came to Huddersfield from Poland in October 2020, with teaching staff describing him as “a pleasant and well-mannered boy” who “was observed to be quiet and timid, and at times appeared sad”.

Prosecutor Jason Pitter KC said that by New Year’s Eve the defendants’ attitude was that Sebastian was “something of a hindrance,” and that from January 2021, “the disciplining of (him) became progressively harsh”.

The court heard his “punishments” were “by any stretch of the imagination cruel and became increasingly more severe and violent over time”.

After the couple were arrested in connection with Sebastian’s death, police seized CCTV cameras from their house, which prosecutors said had been installed partly to “monitor and exert control over Sebastian remotely”.

The footage revealed weeks of abuse endured by Sebastian before he died, with footage played to jurors including Latoszewski beating Sebastian’s buttocks with what appears to have been a slat from a bed, making him do exercises, and whipping him with an extension cable to “spur him on”.

Video from Latoszewski’s phone also contained clips of him verbally abusing the visibly upset teenager.

On the day before Sebastian’s death, Latoszewski was seen on CCTV forcing food and drink into the boy’s mouth and stabbing him a number of times with a needle in the groin and thigh.

Mr Pitter told the trial that Latoszeski was “apparently laughing whilst he did so”, and that Kalinowska at one point “took over stabbing Sebastian with the needle”.

Jurors heard that, on the morning of Sebastian’s final day, August 13 2021, he went through “the routine of being assaulted by both defendants”.

The court heard CCTV showed Latoszewski taking Sebastian out of the bedroom at 8.25am, before carrying him back around 15 minutes later, “naked, clearly wet and unconscious”.

Mr Pitter said emergency services were called to the defendants’ house by Latoszewski after Sebastian had been unconscious for around two and a half hours.

Speaking after the sentencing, West Yorkshire Police senior investigating officer Detective Chief Inspector Tony Nicholson paid tribute to his team, who had viewed about 4,000 hours of harrowing CCTV footage during the investigation.

He told reporters: “I’ve been a police officer for 29 years and I can fairly safely say this case has been the most harrowing and tragic, the most heartbreaking, that I’ve investigated.

“Sebastian was a child who could not have been more isolated and appears to have suffered the most appalling abuse in silence, and even come to see it as normal.

“Latoszewski and Kalinowska abused this boy in a manner which was wicked and evil, and we may never know why they chose to act in this inhuman way.

“We recovered overwhelming evidence of their attacks and of them discussing them in phone messages, and I can say that footage we have seen of this abuse has upset experienced detectives.”