Turkish President Recep Tayyip Erdogan announced Friday that the Hagia Sophia, one of the architectural wonders of the world, would be reopened for Muslim worship, sparking fury among Christian leaders and in neighbouring Greece.

His declaration came after a top Turkish court revoked the sixth-century Byzantine monument's status as a museum, clearing the way for it to be turned back into a mosque.

The UNESCO World Heritage site in Istanbul, a magnet for tourists worldwide, was first constructed as a cathedral in the Christian Byzantine Empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453.

The Council of State, Turkey's highest administrative court, unanimously cancelled a 1934 cabinet decision and said Hagia Sophia was registered as a mosque in its property deeds, in its detailed reasoning seen by AFP.

Greece branded Turkey's move an "open provocation to the civilised world". "The nationalism displayed by Erdogan... takes his country back six centuries," Culture Minister Lina Mendoni said in a statement. The Russian Orthodox Church said Turkey had ignored "millions of Christians" with its move.

'Let the chains break'

Hagia Sophia has been a museum since 1935 and open to believers of all faiths. Transforming it from a mosque was a key reform under the new republic born out of the ashes of the Ottoman Empire.

The landmark ruling will inflame tensions not just with the West and its historic foe Greece but also Russia, with which Erdogan has forged an increasingly close partnership in recent years. The United States has also urged against altering its status.

The UN's cultural agency UNESCO earlier Friday warned Turkey against converting Hagia Sophia into a mosque, urging dialogue before any decision was taken. Ahead of the court decision, Justice Minister Abdulhamit Gul shared a picture of Hagia Sophia on his official Twitter account, with a message: "Have a good Friday."

Finance Minister Berat Albayrak, Erdogan's son-in-law, tweeted that Hagia Sophia would be reopened to Muslim worship "sooner or later", referring to a quote from Turkish poet Necip Fazil Kisakurek.

The Council of State had on July 2 debated a case brought by a Turkish group -- the Association for the Protection of Historic Monuments and the Environment, which demanded Hagia Sophia be reopened for Muslim prayers.

Since 2005, there have been several attempts to change the building's status. In 2018, the Constitutional Court rejected one application. Despite occasional protests outside the site by Islamic groups, often shouting, "Let the chains break and open Hagia Sophia" for Muslim prayers, authorities had until now kept the building as a museum.