The concert, which will take place at Cadogan Hall on Sunday 4 June, will feature an all-star casting including two-time Olivier Award-winner Janie Dee, Olivier Award-nominee Laura Pitt-Pulford and two-time Grammy Award-winner Lucy Schaufer.
The cast will also include David Birrell, Fra Fee, Lauren Morris, Jordan Lee Davies and Tamsin Dowsett.
Celebrating the music and lyrics of Stephen Sondheim, the concert will feature music from works including Sweeney Todd, Sunday in the Park With George, Into the Woods and Follies.
Conducted by Parker and directed by Knights, the orchestra will be led by Emily Davis.
Sondheim: Smiles of a Summer Night plays Cadogan Hall on Sunday 4 June.