Lyttelton [2nd L] has presented the show since 1972
The BBC has cancelled the latest Radio 4 series of I'm Sorry I Haven't A Clue as its presenter, Humphrey Lyttelton, 86, is undergoing major surgery.
Lyttelton, who has hosted the "self-styled antidote to panel games" since 1972, is being operated on to repair an aortic aneurysm.
Producer Jon Naismith told fans: "We are unclear precisely how long Humph's recovery period will be."
He added that "Humph" was "otherwise fine and in good spirits".
Retired from jazz show
Lyttelton appears on the show with Graeme Garden, Barry Cryer and Tim Brooke-Taylor.
In 1993 he received a Sony Gold Award for services to broadcasting.
Lyttelton, once described by Louis Armstrong as "that cat in England who swings his ass off", is also a musician, cartoonist, calligrapher and author.
He began playing the trumpet in 1936 and still tours with his band - best known for the song Bad Penny Blues. In 1956, they were the first British jazz act to enter the top 20.
Lyttelton also added trumpet to Radiohead's Life in a Glasshouse, from the Amnesiac album, although he admitted he "couldn't make head or tail" of the track when he first heard it.
The former officer in the Grenadier Guards recently retired from BBC Radio 2's The Best of Jazz, to "clear a space for some of my other ambitions" after more than 40 years presenting the show.