Flavio, Rè di Longobardi was first produced at the King's Theatre in London's Haymarket on May 14, 1723. It was the fourth full-length opera composed by Handel for the Royal Academy of Music, an organization founded in 1719 to promote Italian opera. Handel had been closely involved with the Academy since its inception, having already had Radamisto (1720), Floridante (1721), and Ottone (1723) produced under its auspices. The libretto by Nicola Francesco Haym, who also collaborated with Handel on Ottone, is an adaptation of an older Venetian book involving elements of Corneille's tragicomedy Le Cid; the plot centers on a dispute between two counselors, who are here attached to the court of Flavio, a legendary king of medieval Lombardy. A romantic imbroglio involving the son and daughters of the counselors is ultimately resolved by Flavio. As in Floridante, Handel treats the story with an overall lightness of touch and with a mixture of genres that defies the usual pigeonholing of his Italian dramatic works as opere serie. The Venetian origin of the plot doubtless encouraged Handel to introduce the elements of tragedy, comedy, parody, and satire, all of which played parts in the tradition he had already successfully exploited in his first great opera, Agrippina, produced in Venice in 1709. The mixture of styles and the subtlety of the interaction between the two elderly counselors and the two sets of lovers has indeed led the great Handel scholar Winton Dean to draw analogies between Flavio and Mozart's Così fan tutte.

The opera is cast in the usual three acts, largely following the characteristic formula of alternating recitative and da capo aria. However, the opera opens with a duet for one of the pairs of lovers, the coquettish Teodata and her secret amour Vitige;this is complemented near the end by another for the more serious romantic couple, Guido and Emilia. The latter part was created by one of the great prima donnas of the day, Francesa Cuzzoni, who had joined the Royal Academy company the previous year, and for whom Handel composed some of the loveliest arias in the opera. The sad "Parto, sì" from Act II in particular is of great beauty, the voice joining in dialogue with the upper instruments to magical effect. In keeping with the spirit of the opera, the scoring is mostly delicate and calls for an orchestra in which the strings are joined only by oboes, a flute, and a recorder. In addition to Cuzzoni, the original cast also included the great castrato Senesino, who sang the part of Guido. Despite such stars the opera was not a success, receiving a run of eight opening performances. It was subsequently revived by Handel only once, in 1732. The neglect of the work has largely continued even in an era devoted to the revival of Handel operas -- perhaps, as Dean suggests, because it refuses to conform to what is expected of a Handel opera.