Jimmy Osmond - who became a child star in the 1970s with The Osmonds - has been diagnosed with a stroke.
The 55-year-old US entertainer was taken to hospital after appearing as Captain Hook in the production of Peter Pan at the Birmingham Hippodrome on Thursday.
A spokesperson said he had completed the performance and signed autographs for fans before going to hospital.
Osmond is now taking time out to recover and will not return to the production, the spokesperson said.
In a statement, on Monday, the spokesman said: "On the evening of Thursday December 27 after pushing through the evening's performance of Peter Pan at Birmingham Hippodrome, Jimmy Osmond was driven straight to hospital and diagnosed with a stroke.
"He is grateful for all the well wishes and will be taking time out in the new year."
Birmingham Hippodrome's artistic director and chief executive Fiona Allan said: "Everyone here at Birmingham Hippodrome has been deeply saddened to hear of Jimmy's sudden illness.
"Jimmy loved being a part of the Hippodrome's well-renowned panto and his portrayal of Captain Hook was both dastardly and heart-warming.
"He won the adoration not just of our audiences, but also of all our staff - we all send Jimmy and his family very best wishes for a speedy recovery."
The theatre added that Darren Day would be replacing Mr Osmond in the role - currently being covered by his under-study, Luke Redford - from later in the week.
