Blonde Holly Scally, who dated Niall before he found fame with the boyband, was the victim of a sick joke on the micro-blogging site, where one user claimed she had passed away from cancer.
The user wrote: ''Guys, I have some really bad news, Niall's ex-girlfriend Holly has passed away due to cancer.''
Later the topic ''RIPHollyScally'' spread across the site, leading to a furious Holly to lash out.
She wrote on her own page: ''Apparently I'm dead. Aah.
''Seriously all you f***ing one direction fans you need to f**k off and make up some rumours about someone else! I'm not dead or pregnant. Bye now c.
''I have a boyfriend who is A LOT better than Niall f***ing Horan :L Shut up and get a life (sic).''
A source close to Niall said 18-year-old teenager will be horrified by the claims.
The insider told the Irish Sun newspaper: ''This is just some sick hoax - Holly is absolutely fine. She went out with Niall for a while years ago but now has girls giving her abuse online every day of the week.
''Niall won't be happy to hear how far some of these fans have gone with someone who knew him before all of this.''
Niall and his bandmates - Harry Styles, Louis Tomlinson, Liam Payne and Zayn Malik - are currently enjoying worldwide success with their debut album 'Up All Night'.
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