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The siblings will be unable to take their regular role at the ceremony because of TV commitments in America, according to The Sun.
"The only reason why Kelly and Jack are not hosting the ITV2 show is because they are in LA," said a source.
Kelly, 23, and Jack, 22, are currently working on a variety show with parents Sharon and Ozzy.
However, NTAs executive producer Kim Turberville insisted that this year's backstage presenters, Sara Cox and Scott Mills, were their first picks for the job.
"We like to rotate presenters to keep the show fresh, and in the past Tess Daly and Fearne Cotton have both hosted the ITV2 show," she said.
"Having listened to Sara Cox and Scott Mills on Radio One, they were our first choice this year, and we're absolutely thrilled they'll be presenting our ITV2 live coverage after the ceremony."
Trevor McDonald will helm the main awards ceremony at the Royal Albert Hall.
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