SACRED choral music at Oxford was dominated for decades by three choirs: those of Christ Church, Magdalen, and New College, each with its own choir school.

True, there were others - the all-male choir of St John's drew its boys from city schools; Exeter, Keble, Lincoln, and several of the women's colleges flourished with choirs run by their respective undergraduate organ scholars. But the big three, overseen by charismatic choirmasters of exceptional gifts, such as Bernard Rose, Simon Preston, and Edward Higginbottom, ruled the roost.

Until now. As the recent "Passiontide at Merton" festival demonstrated, Oxford now has a fourth college, with women on the top (treble and mean) lines, that is easily a match for the top trio. In the Michaelmas term 2008, Peter Phillips launched the new Merton College Choir, like Trinity and Clare in Cambridge richly endowed with choral scholarships. In less than five years, Phillips has established it as a musical force of astonishing quality and character.

Its young singers are sophisticated, repertoire-familiar, energised, and empathetic to music of all periods. To hear an evensong there is to be transported. Musically speaking, Merton has arrived.

The current co-director of music, who devised and conducted the festival's main events, is Benjamin Nicholas, son of a former organist of Norwich Cathedral, and a vitalising former Oxford organ scholar, who more recently transformed, and saved, the excellent boys' choir of Tewkesbury Abbey.

Inspiring and yet unostentatious, Nicholas, with his vocal charges Carys Lane and Giles Underwood (a formidable Cambridge ex-choral scholar), has trained his Merton (mostly) undergraduates to an astonishing degree of accuracy and flair: their vowels are of al- most unique quality for such an ensemble; their delivery is exciting, sensitive, commanding, fluent; they are, and sound, meticulously rehearsed.

You could hear all this in the new evening canticles specially commissioned for the choir's 750th-anniversary Merton Choirbook from the Latvian composer Eriks Esenvalds (b. 1977). The setting is relatively straightforward, but its reliance on shifting dynamics produced some unusual fireworks.

Merton's magnificent countertenors get the star parts: at "He hath shewed strength with his arm", a passage more usually associated with bass voices; and in the glorious, melisma-tinged countertenor solo cantilevering out of the textures of the Gloria.

Esenvalds's new Nunc Dimittis is less clustering (apart from "Which thou hast prepared"), more tonal, but rhapsodic; and here the rising tenors ("To be a light . . .") took the honours. But it was Matthew Martin's Responses and a sensational rendering of Psalm 130 ("Out of the deep") which deserved the limelight. The chant's composer (here Purcell, adapted by Turle) should surely be identified in any song sheet.

The main item (a polished Handel's Messiah apart) was Estonian Arvo Pärt's St John Passion. Here was the most compelling performance I have heard of this plaintive, patient, sombre work, a match even for the Hilliard Ensemble's famous recording. The young countertenor who has the lion's share of an excellent solo quartet possessed a beauty out of this world. The choir's recurrent interjections were all spot on. And if Christopher Borrett, the bass singing Jesus - his placing in the antechapel (in the organ lift) was acoustically imaginitive - seemed sometimes just off-note and less than dramatic (admittedly, that is part of Pärt's point), his opposite number, Timothy Coleman, the tenor Pilate, rang out with all the benefits of the building's expressive acoustic.

Beforehand, Meirion Bowen, the brains behind the annual Cheltenham Festival, delivered an engaging, informative talk on Pärt, the musical examples ingeniously illustrating the composer's pilgrimage from Soviet-era modernist via the plaintive elegy Für Alina to his current bell-like ("tintinnabular") vocabulary and manner.

All this was preceded by a sequence of English composers for voice and piano. What impressed was not so much Britten's Canticle Abraham and Isaac, enchantingly sung (the unison touches not least impressive) by the alto Jeremy Kenyon and the tenor Thomas Elwin (an ex-head chorister of St Paul's Cathedral), or even four exquisitely accompanied Britten folksongs, but rather some Purcell solo songs realised by Britten ("Evening Hymn", culminating in a touching Alleluia), if not always beneficially. The music at "drop, drop, drop" ("Music for a While") evidences Purcell's sensational response to the words he sets, in the same class as Gibbons and Walton.

The plum was Kenyon's singing, with his (here) light-touch accompanist Libby Burgess, of Tippett's three Songs for Ariel, penned for a 1962 staging of The Tempest (a play for which Purcell also wrote music). Dark, delicate, impish ("Where the bee sucks"), buoyant, trumpetingly triumphant, these songs typify the best of Tippett, and capture to perfection the best of Shakespeare. With such a Messiah to boot, Merton's Passiontide weekend proved a mellifluous triumph.