WHAT is believed to be the world's first "Wrenathon" - almost 17 hours of choral music, sung by 17 choirs in 17 London churches designed by Sir Christopher Wren - is to take place tomorrow.

The Wren Choral Marathon starts at 8 a.m. in St Michael's, Cornhill, where the National Youth Chamber Choir of Great Britain will perform. It concludes at midnight with vespers at St Stephen Walbrook, sung by Khoros, a group of 35 young adults from south London.

Other venues will include St Paul's Cathedral, St Mary-le-Bow, and St Andrew by the Wardrobe.

The performances will feature a variety of professional, amateur, and children's choirs, from the London Gay Men's Chorus to the vocal ensemble Tenebrae, the Singaporean a cappella group MICappella, and the London Soul Choir.

They will perform a range of musical styles: medieval, classical, contemporary, gospel, and jazz. Each programme lasts 45 minutes, which will give the audience 15 minutes to travel to the next venue.

Those hardy enough to manage all 17 events can have their programmes marked at each church, and be entered in a prize draw.

The marathon, part of the City of London Festival, was devised by its director Paul Gudgin, who said: "I became aware of what a unique feature it is to have so many churches created by a single architect in such a relatively confined area. I felt we had to find a way for the festival to highlight this remarkable legacy in an original way."