The world premiere is taking place of a work by Welsh composer Joseph Parry written almost 150 years ago.
Te Deum will be performed at London's St Giles Cripplegate church by the London Welsh Chorale (LWC) on Saturday.
A text from the Book of Common Prayer, Te Deum was set to music by the young Parry in 1863 while in America.
Conductor Edward-Rhys Harry said the work had been lost when pieces by Parry, such as Myfanwy and Aberystwyth, had become known internationally.
Mr Harry uncovered Te Deum during his research studies into the roots of the choral tradition at Royal Welsh College of Music and Drama.
"It was Parry's setting of the text that I discovered as part of a collection of manuscripts, signed by the man himself as being completed in Danville, Pennsylvania, in 1863," he said.
There is no record of Te Deum being performed anywhere in the world.
The performance forms part of the chorale's 30th anniversary celebrations and its president, the former Labour leader Lord Kinnock, said they could not have chosen a better centre piece.
"The Welsh connection is obvious and, of course, appropriate. But it's typical of the chorale to be adventurous and break new ground with what must be a demanding musical work.
"I'm sure that it will have the best of Parry's great resonance and the chorale will certainly give it everything they've got. "
Mr Harry said the chorale was "buzzing with excitement" at the prospect of performing a world premiere as part of its 30th anniversary celebrations.
"The LWC are in a unique position, being able to promote the music of Wales and Welsh composers outside the borders of their own country," he added.
He has spent the last year transcribing the music from Parry's handwriting into a readable format, revising some of the musical errors and providing an orchestration for it.
Parry, who was born in Merthyr Tydfil in 1841, emigrated to Danville in 1854. He returned to Britain in 1868, and became music professor at the University of Wales, Aberystwyth, in 1872.
His best known work, Myfanwy, was published in 1877. It has become a staple of male choirs' repertoires and provided the close to the 1997 Swansea-based film Twin Town.
His hymn tune Aberystwyth was so popular in his lifetime that in 1897 a South African Methodist teacher, Enoch Sontonga, wrote a hymn to its melody.
Nkosi Sikelel' iAfrika became the national anthem of Zambia, Tanzania, Namibia and Zimbabwe after independence, and is also co-national anthem of South Africa.
Parry died in Penarth, Vale of Glamorgan, in 1903.