Premiering later this week, Littner will only appear in two episodes and in the others be replaced by former Apprentice winner Tim Campbell.
Speaking to the Radio Times, Sugar confirmed that Littner's role was reduced due to "medical issues" believed to be from a bicycle accident the businessman suffered in 2021.
Of Campbell taking over the role, Sugar said: "Obviously it's been a great journey starting with [former aide] Nick Hewer and then Claude. Unfortunately, Claude suffered some medical issues and so I was asked to think of someone else – and who better than somebody who's actually been through the process."
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Sugar added: "So it was a bit of a no-brainer really, because when it comes to making excuses like these candidates do, you've got Tim there, who's been there, done that, seen it and heard it all.
"And that's why I warn the candidates also that there's no pulling the wool over his eyes, so to speak."
Littner joined The Apprentice as Lord Sugar's aide in season 11 following the departure of Nick Hewer. In the previous 10 seasons, Littner had acted as one of the interviewers during the later stages of the competition.
Since winning the first season of The Apprentice in 2005, Campbell went on to work for Sugar until 2007. He has since set up his own social enterprise Bright Ideas Trust and sits on the City of London education board.
He received an MBE in 2012 for services to enterprise culture.
Watch below: On this Day 1986: Alan Sugar expands his tech empire.