Singer Lesley Garrett and lyricist Tim Rice are among the guest hosts lined up to present Countdown while Richard Whiteley recovers from pneumonia.
Whiteley has presented Countdown for 23 years
Host Whiteley will miss a recording for the first time in the Channel 4 quiz show's 23-year history after being admitted to hospital on 6 May.
Broadcaster Esther Rantzen and film critic Barry Norman are among the others due to step in.
Each guest host will present five shows, beginning with 4 July's edition.
Comedian Richard Digance, broadcaster Nicholas Owen and former BBC Royal correspondent Jennie Bond complete the list of stand-ins.
All the guest hosts have previously appeared on the quiz show.
Whiteley, 61, is still on screen as episodes are pre-recorded.
He was rushed to Bradford Royal Infirmary after falling ill. He is said to be "recovering slowly" from his illness.
He has presented Countdown with Carol Vorderman since 1982.
Vorderman will continue in her usual role of co-presenter on the show, which has regular viewing figures of four million.
She will also record a special message for viewers at the start of the first show, to explain Whiteley's absence.
She said the Countdown team were "thinking of Richard and wishing him a speedy recovery".
"I know I'll miss Richard terribly on studio days and I am nervous about hosting the show with anyone else," she said.
"However, I'm really pleased that the guest presenters who are in Richard's chair are already members of the Countdown family and have been on the show many times before."
Channel 4 head of daytime Adam MacDonald: "We're grateful to our guest presenters for stepping in at short notice and am sure they will do a great job of keeping Richard's chair warm for him until he returns."
Last November Whiteley was honoured with an OBE from the Queen, who is said to be a fan, as was the late Queen Mother.