The World Council of Churches has called on Turkey's President Recep Tayyip Erdogan to reverse his decision to turn the celebrated Hagia Sophia museum back into a mosque.
In a letter to Mr Erdogan, the Council, which counts 350 churches as members, said the move would sow division.
The Unesco World Heritage site in Istanbul has been a museum since 1934.
The president announced his decision on Friday following a court ruling which annulled its museum status.
The Hagia Sophia was built 1,500 years ago as an Orthodox Christian cathedral, but was converted into a mosque after the Ottoman conquest in 1453.
It was converted to a museum on the orders of Mustafa Kemal Ataturk, the founding father of modern, secular Turkey.
Since then religious services have been banned at the site, but devout Muslims have long campaigned for worship to be allowed.