Ant McPartlin and Declan Donnelly have pulled out of work engagements after falling ill.
The pair, who are gearing up to host another series of I’m A Celebrity, reportedly have a ‘sudden’ illness, which has forced them to scrap an appearance on a Britain’s Got Talent special.
They will be replaced on BGT by former Britain’s Got More Talent host Stephen Mulhern, an ITV spokesperson said.
There is also now a question mark over whether they will attend tomorrow’s National Television Awards.
The pair have won the Best TV Presenter Award for the past 20 years.
The duo were set to present a two-hour special, Britain’s Got Talent: The Ultimate Magician, which would involve magicians from past series competing for the title of ultimate champion and £50,000.
An ITV spokesperson said: "Ant and Dec are both unwell and have postponed all work commitments until they recover.
"Sadly, this means they were unable to film Britain’s Got Talent: The Ultimate Magician.
“Whilst we wish them a speedy recovery, we are thrilled to confirm that BGT’s favourite magician, Stephen Mulhern, has stepped in."
Judges Amanda Holden, Alesha Dixon and David Walliams will make a return, but Simon Cowell will not as he has filming commitments in the US.
He will be replaced by world-famous magician Penn Jillette, one-half of the Penn & Teller team.
The BGT special was filmed in front of a live studio audience on Tuesday and will involve 10 acts.
It is not yet known when the show will be broadcast.