Broadcaster Alan Titchmarsh and Grantchester actress Tessa Peake-Jones will take to the stage when they appear on the All Star Musicals show.
Emmerdale actress Amy Walsh and Coronation Street star Daniel Brocklebank are also due to join the one-off ITV programme, which pays homage to the West End and Broadway.
Financial journalist Martin Lewis, comedian London Hughes and Joel Dommett, who hosts I’m a Celebrity: Extra Camp, have also been announced.
Titchmarsh, a gardener, presenter, poet and novelist, said his on-stage experience extended only to singing in the church choir.
The broadcaster added that while he had always steered clear of reality television, this offer was too good to turn down.
He said: “I’ve always loved musicals and while I have always resisted taking part in any ‘reality’ TV shows I found the chance of being able to sing on the stage of the London Palladium irresistible.
“That said, I’m now scared stiff. I sang as a boy in the church choir and later as an adult, but I have never sung anything like this in public. Eeek!”
Peake-Jones is best known as Del Boy Trotter’s wife Raquel in Only Fools And Horses.
More recently she played Mrs Sylvia Maguire, the vicar’s cantankerous housekeeper in Grantchester.
She said: “I was keen to do this because I wanted to be shaken out of my comfort zone and boy am I being shaken.
“I’m really excited about the Palladium and being part of a team musically with an ensemble as I’ve never had that experience.”
Elaine Paige, dubbed the first lady of musical theatre, will lead the panel of judges.
Also on the panel are Strictly Come Dancing’s Kevin Clifton, US singer Kristin Chenoweth and West End star Trevor Dion Nicholas.
Paige, 70, will also mentor the seven celebrities until they perform a musical standard or well-loved modern classic at the London Palladium.
The programme will also feature performances from host John Barrowman and the panel, with Paige and Chenoweth performing a duet for the first time.
All Star Musicals returns to ITV this spring.