The Good Morning Britain host started the spat by slamming the reality TV star after reports suggested she’s the world’s youngest self-made billionaire.
Piers, 53, insisted Kylie, 21, only got rich because her sister Kim Kardashian made a sex tape, which shot her to fame.
He tweeted: “It’s not self-made, it’s because her sister made a sex tape.”
Liam, 25, responded by defending Kylie, writing: “I’m sorry but piers are you crazy your only here cuz you talk s**t about everyone and make controversial outbursts about people to get attention…
Liam’s tweet left Piers furious and he snapped back by slamming the singer and correcting his punctuation.
The telly star wrote: “Hi Liam, a) She didn’t ‘build herself’, she became rich & famous from her sister’s sex tape. b) It’s ‘you’re’.”
The feud rumbled on as Liam took the bait and snapped back: “Sorry could you say it louder from the back of the line of irrelevant people please.”
Piers continued to mock Kylie, calling her a bad role model for young girls because the family made their fortune from “sex tapes and topless selfies”.
He then made another jibe at Liam.
When a One Direction fan tweeted to Piers: “Did you really just say that to the best vocalist of our generation?” Piers replied: “Liam’s not even in the Top3 vocalists in One Direction.”
Even Piers’ son Spencer got in on the Twitter action, defending his dad by posting a tweet mocking Liam.
Earlier this week, Forbes magazine reported that Kylei had become the world’s youngest self-made billionaire of all time.
Her success was put down to her savvy use of social media, her family’s position as the most-watched people in America, and her global makeup brand Kylie Cosmetics.
Her net worth now far outstrips the rest of her family – momager Kris Jenner, supermodel sister Kylie Jenner and her reality star siblings Kim, Khloe and Kourtney Kardashian, as well as dad Caitlyn Jenner.