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He joined the Sadler's Wells Theatre in 1968, and the Royal Opera House in 1970. He was also a regular guest at the English National Opera and the Welsh National Opera. His roles have included most of the major bass roles of the Verdi and Wagner repertories, such as: Miller, Sparafucile, Ferrando, Padre Guardiano, Phillip II, Landgrave, Hans Sachs, Gurnemanz, etc. In his seventies, he continues to sing small roles on stage: Simone in Gianni Schicchi at the Royal Opera House in 2009 and 2016, and Schigolch in Lulu at the Metropolitan Opera in 2010.
Howell took part in the premieres of two of Peter Maxwell Davies's works, Taverner and The Doctor of Myddfai. He can be heard in studio recordings, as Jero in L'assedio di Corinto, Count Walter in Luisa Miller, and as Capulet in a live recording of I Capuleti e i Montecchi, opposite Agnes Baltsa and Edita Gruberova, under Riccardo Muti, at Covent Garden, in 1984.