The chilling confession tapes of Britain's youngest double murderers have been released.

On April 15, 2016, Kim Edwards and her boyfriend Lucas Markham, both just 14 at the time, murdered Kim’s mother and younger sister.

The bodies of Elizabeth Edwards, 49, and her daughter Katie, 13, were found in their Spalding, U.K. home.

Kim and Lucas were arrested and charged with the killings, making them the country’s youngest double-murderers.

They were quickly dubbed the “Twilight killers” after it was revealed that after the pair stabbed Elizabeth and Katie ten times as they slept, they had sex, cleaned themselves up, ate tea cakes and ice cream, and watched the movie Twilight.

Kim and Lucas' identities were only revealed to the public earlier this month after they were found guilty of the two murders and handed life sentences. They will both spend at least 20 years behind bars.

Now, Lucas' chilling confession to police about his role in the murders has been released to the public, as part of a new Channel 5 documentary, Murdered By My Daughter, which aired in the U.K. last night.

According to The Sun, the documentary reveals Lucas was angry that Kim's mother had tried to end the young couple's relationship.

In audio obtained by the documentary, Lucas tells police in a "calm" voice exactly how he carried out the murders.

"I went into her mum's room and stabbed her in the neck while she was asleep on her side and smothered her face with a pillow," he told police.

"And after I knew she had gone, I went into Katie's room—which is the same room as Kim's—and I thought I stabbed her, but...I thought I stabbed her, but I’m not a hundred percent sure. It was, like, her on a mattress and then I smothered her face with a pillow too."

He admitted he only killed 13-year-old Katie because he was afraid she would tell police that he and Kim were responsible for her mum's death.

Earlier in June, police released audio from Kim's confession. She confessed her motive for the murders was that her mum "favoured my sister more than me."

Kim also revealed that she had very little remorse over the crimes and that she had "made peace" with killing her mum and sister.

"It wasn't like torture or anything," she told police during the interview.

"Just, the fact that it happened so quickly gave me peace of mind," she said, adding that "in a way" she was happy they were dead.

"My mum doesn't have to deal with me anymore, being like, you know, suicidal, and she doesn't have to wake up worrying every morning to see if I'm still alive.

"And my sister doesn't have to go through all the heartbreak and all the emotions and stuff."

Family of the victims have slammed the Channel 5 documentary, with Elizabeth's eldest daughter Mary claiming the show had "dragged her mother's name through the dirt."

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"I'm disgusted that we weren't able to have any involvement in the documentary. My family and I weren't happy with it at all, it was all complete rubbish," the 27-year-old mother-of-three told the Daily Mail.

"I feel like they were dragging my mum's name through the dirt and she's not here to stand up for herself.

"They didn't ask for our permission. They told us they were going ahead with it and there was nothing that we could do. It is too soon to be dragging us through this."