They were first linked to each other back in May when she was dating Made In Chelsea co-star Andy Jordan.
But Niall Horan and reality star Louise Thompson are said to have stepped up their rumoured romance in recent weeks after enjoying a series of clandestine meetings at the One Direction hunk's North London home.
While the diminutive brunette initially denied any involvement with the boyband heartthrob, she was forced to admit cheating on Andy during a recent episode of Made In Chelsea.
And since splitting from her co-star beau, Louise is said to have spent the night at Niall's home in the capital on numerous occasions over the past few months.
A show source told the Sun On Sunday newspaper: 'Louise has been dating Niall - they've seen each other lots and talk all the time. But it suited both of them for their nights together to be secret. He'll sneak her into his house in North London, often when it's dark.
'She makes sure she doesn't do anything around Chelsea to try and keep the romance away from the prying eyes of her co-stars on the show.'
Andy was left heartbroken on Monday night's episode of the E4 reality show when Louise's womanising former flame, Spencer Matthews, claimed to have slept with the geography student after a recent dinner party.
But news of her alleged fling with Niall is said to have left him even more dismayed.
The source added: 'Louise always insisted to Andy that she was just friends with Niall and that she;d only spent one night at his house and it was completely innocent. Foolishly, he believed her because he was madly in love.
'But he's now aware of the fact Louise and Niall are in a passionate physical relationship and hve been together on numerous occasions.'
Louise broke down in tears on a recent episode of Made In Chelsea when she was forced to admit to sleeping with a 'guy' - believed to be Niall - during a showdown with a devastated Andy.
While the denim entrepreneur has had long-term romances with co-stars Andy and Spencer, Niall was most recently linked to 16-year-old X Factor contestant, Tamera Foster.
A representative for Louise Thompson declined to comment on the latest claims when approached by MailOnline.
MailOnline has contacted a spokesperson for Niall Horan for comment.
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