The cause of Kim and Kourtney Kardashian’s headline-making fight has been revealed.

Trailers for the family’s reality TV show had teased the warring sisters coming to blows, with punches and kicks thrown during an expletive-filled row.

The season 18 premiere of Keeping Up With The Kardashians has now aired in the US, showing fans what sparked the brawl.

And it was revealed as a dispute over Kourtney’s workload, with Kim hinting her older sister – who has considered quitting the show for a more private life – was lazy.

It began during a discussion about Kylie Jenner cancelling her role in a fashion show after falling ill. Kim told sisters Kourtney, Khloe and Kendall Jenner she would have done the show on her “death bed” before insinuating the others would not do the same.

Kendall, a supermodel, took offence when Kim hinted her “anxiety” stopped her from working as hard as her. She then implied Kourtney was lazy.

A furious Kourtney shot back: “You act like I don’t do shit! You have this narrative in your mind…I will literally f*** you up if you mention it again.

“But, honestly, change the narrative in your mind. I work my f***ing ass off.”

She then threw a drink at her before they exchanged blows.

“What the f*** is wrong with you?” a stunned Kim said “Don’t ever come at me like that. I swear to God, I’ll punch you in your face.”

As Khloe tries to intervene, another punch is thrown before they scratch each other. The footage ended with Kim slapping Kourtney in the face.

After the episode aired, Kourtney made light of the fight and tweeted: “Kourtney ‘the nail digger’ Kardashian weighing in at 108 pounds…Kim ‘the kicker’ Kardashian West weighs in at 128 pounds…”

Earlier in the episode, it was revealed Kris Jenner, the family matriarch, had offered Kim 400 thousand dollars (£325,000) to replace the ill Kylie during a Paris Fashion Week show.

The younger sister was forced to back out of an agreement with the Balmain fashion house after coming down with strep throat.

Kris failed in her bid to entice Kendall to walk instead and was forced to make a huge offer to Kim, who at first turned down 300 thousand dollars (£243,938).

“Okay, she’s upped it,” Kim said while reading her mother’s text. “400k now. I won’t take a commission. The jet can leave tonight. You can bring North.”

Season 18 of Keeping Up With The Kardashians will premiere on Sunday March 29 at 9pm on E! in the UK.