The World Council of Churches has written to Turkey’s president expressing “grief and dismay” over the decision to change the status of Istanbul’s landmark Hagia Sophia from a museum to a mosque.

Interim secretary general of the council Ioan Sauca wrote that, as a World Heritage museum, “Hagia Sophia has been a place of openness, encounter and inspiration for people from all nations”.

The colossal Hagia Sophia was built 1,500 years ago as an Orthodox Christian cathedral and was converted into a mosque after the Ottomans conquered Constantinople, now Istanbul, in 1453.

The secular Turkish government decided in 1934 to make it a museum.

Mr Sauca said the museum status had been “a powerful expression” of Turkey’s commitment to inclusion and secularism.

Turkish president Recep Tayyip Erdogan has formally converted the building back into a mosque and declared it open for Muslim worship, hours after a high court annulled the 1934 decision to turn it into a museum.

Mr Erdogan, a devout Muslim, has frequently used the debate over Hagia Sophia to drum up support for his Islamic-rooted party.

The decision has provoked deep dismay among Orthodox Christians.