As Carmen, Amneris in Aida, Marina in Boris Godunov, or Herodias in Salome, she was never less than admirable musically and plausible dramatically, but her two finest roles were undoubtedly Brangane in Tristan and Isolde and Octavian in Der Rosenkavalier, both of which she sang under the magic baton of the Austrian conductor Erich Kleiber.
It must have been daunting for a singer of less than two years' operatic experience to tackle Brangane with the great Kirsten Flagstad as Isolde, but Shacklock did so to the conductor's satisfaction and he later invited her to sing the role in Berlin. She also recorded part of the second act with Flagstad, and Set Svanholm as Tristan. Her voice rings out splendidly in Brangane's warnings from the tower.
As Octavian she managed to suppress the usual warm femininity of her voice and personality sufficiently to pass muster as a young man, while the sincerity of her acting was particularly remarkable in scenes such as the Presentation of the Rose. She was also very funny in the last act when Octavian is disguised as "Mariandel".
Born at Sherwood, near Nottingham, in 1913, Shacklock sang as an amateur with local concert and opera societies. After studying for four years in London at the Royal Academy of Music with the baritone Frederic Austin, she made her professional debut in 1944 as a concert singer. In 1946 she joined the Covent Garden Opera, and took part in the production of Purcell's The Fairy Queen with which the fledgling company began operations. The following year she sang Mercedes in Carmen and Annina in Der Rosenkavalier, but soon graduated from these minor roles to Carmen and Octavian.
During the 1947/48 season she also sang Marina, Azucena in Il trovatore, Magdalene in The Mastersingers and Brangane. The following season she added Amneris and Mrs Sedley to her repertory. In 1949-50 her new roles were Herodias and Fricka in Das Rheingold and Die Walkure. Her other Wagner roles included Erda in Siegfried, First Norn in Gotterdammerung, and Ortrud in Lohengrin. All these operas were, in the first few years of the company's existence, sung in English. Shacklock's extremely good diction was always an additional pleasure in her performances.
During the Coronation season of 1953, Shacklock sang the title role of Britten's Gloriana in alternation with Joan Cross, who created the part. Shacklock was vocally excellent as Queen Elizabeth I, but could not quite match the dramatic weight wielded by her senior colleague. She remained a member of the Covent Garden company until 1956 and continued to sing there as a guest artist until 1960. But times were changing; more and more operas were being sung in the original language.
In 1961 Shacklock joined the cast of The Sound of Music which ran for over 200 performances at the Palace Theatre in London; she sang the Mother Superior. After retiring from the opera stage, she returned to the Royal Academy of Music, where she taught singing until 1984.
Constance Shacklock, opera and concert singer: born Sherwood, Nottinghamshire 16 April 1913; Professor of Singing, Royal Academy of Music 1968-84; OBE 1971; married 1947 Eric Mitchell (died 1965; one daughter); died London 29 June 1999.