"MARGOT LA Rouge," a brief, violent opera about a French prostitute, is one of the most unusual works of the eminent British composer Fredrick Delius. Until recently, it was one of the most neglected -- unperformed for 80 years. Now available on single LP record (Arabesque 8134-L)"Margot" recalls a true story as dramatic and touching as its own tense libretto.

In the early 1930s, at his home in a village near Paris, Delius was slowly dying--blind and paralyzed as a result of a syphilitic infection probably contracted in Florida nearly a half-century earlier. He could no longer engage in the physical activity of writing music, but he continued to compose with the aid of Eric Fenby, a young admirer who had offered to serve as his amanuensis.

The two collaborators, sometimes reworking material Delius had composed earlier, produced a remarkable stream of music in the composer's final years, including the Third Violin Sonata, "A Song of Summer," the "Songs of Farewell," the "Fantastic Dance," a prelude for his 1892 opera "Irmelin" and the "Idyll" for soprano, baritone and orchestra, based on texts by Walt Whitman: "Once I passed through a populous city . . . "

For the "Idyll," Delius and Fenby adapted material composed nearly 30 years earlier for "Margot," an opera that never reached the stage. Written for a competition that it did not win, the opera apparently left a scar in the composer's memory. Delius never referred to "Margot la Rouge," recalls Fenby (who is now the president of the Delius Society in England) even when he was borrowing material from it for his final composition. "We used Delius' full score, beautifully written in his own hand," Fenby adds, "maintaining the original orchestration but reshaping the vocal lines where required."

Now, Fenby wishes he had "read through Delius' autograph score of 'Margot la Rouge' more often." When the BBC decided to produce the world premiere of the opera last year, the only copy available was a manuscript score for voices and piano, and Fenby was asked to reconstruct the original orchestration. Some moments in his reconstruction "may be more dramatic than Delius' original," he confesses, "but my zeal might be heard as a pardonable liberty."

"Margot la Rouge" was finally brought to life after 80 years in limbo, first in the BBC broadcast with the original French text and just last week in its first staged performance by the Opera Theatre of St. Louis. It has been well worth the wait. Composed in the verismo style that was in vogue at the turn of the century, it is not likely to achieve the kind of popularity enjoyed by "I Pagliacci" and "Cavalleria Rusticana," the two short operas in that style that have found a secure place in the standard repertoire. But it might provide a useful answer to a question that constantly plagues opera companies: what to do with the rest of the evening when you have decided to produce Puccini's "Gianni Schicchi." Like that great, short comic opera, it does not need a chorus but it does have a large number of solo singers--many with only a few lines--who could be used in both one-acters.

"Margot la Rouge" is a brief, vigorous excursion into those staples of verismo, sex and violence. It is also, at its touching and musically effective climax, a tender tribute to lost innocence. The story takes place at in a Paris cabaret frequented by prostitutes. Three soldiers come in to escape a sudden thunderstorm, and one of them, Sgt. Thibault, finds his attention drawn by a woman, Margot la Rouge. Slowly, it dawns on him that she is his old sweetheart, Marguerite, who disappeared from their small village five years ago when she was 17. In an ecstatic duet, he persuades her to return with him to their village, "far from evil desires and wicked people." Then Margot's current lover enters; there is a fight in which Thibault is stabbed to death, and Margot, taking his sword, kills her lover.

The libretto, whose author is unknown, works very efficiently in its short span, creating an appropriate atmosphere and sketching as much character and plot complication as the situation requires. The music, dating from a period that many Delius fans consider his vintage years, exploits effectively the contrast between the opera's raffish milieu and the idealism of its climactic duet. It is composed with a fine sensitivity to the dynamics and inflections of the French language. And it has the sound of Delius both in its thematic material and in its orchestration, whether Fenby's memory from 50 years ago or his knowledge of the composer's style deserves the credit. In some segments, of course, the orchestration is already known from the "Idyll," and those segments fit seamlessly into the flow of the opera.

Norman Del Mar conducts the BBC Concert Orchestra and 11 British singers in an enjoyable, idiomatic performance. The voices (including soprano Lois McDonall and tenor Kenneth Woollam in the principal roles) are serviceable, if not of international star quality. They sing in a BBC sort of French that is quite correct even if it would not fool a native French-speaker about their place of birth.