Mr Erdogan signed a decree transferring the 6th-century Hagia Sophia, which had bean a museum for nearly 90 years, to the Diyanet, the country’s highest religious authority.
The decree came shortly after Turkey’s highest administrative court, annulled a 1934 decision to turning the iconic building into an interfaith museum.
The Unesco world heritage site began life as a cathedral, before being transformed into a mosque in the 15th century and later designated a museum under Turkey’s secularist founder Mustafa Kemal Ataturk.
It remains unclear whether converting the architecturally and historically significant building into mosque would result in structural changes – or even new rules for visitors.
Turkey’s President Recep Tayyip Erdogan has ordered the conversion of one of the world’s most treasured cultural sites into a Muslim house of worship, potentially sharpening the rift between the Ankara government and Europe.
Istanbul is full of architecturally significant mosques and cathedrals that draw tourists as well as the faithful.
Worshippers have been allowed to pray in a certain part of the museum since 1991. In 2015, a Muslim cleric recited the Quran inside the Hagia Sophia for the first time since it was designated a museum.
But the move has already angered historic preservationists, members of the Eastern Orthodox faiths and western religious freedom advocates. The site is also a major tourist draw, among the most cherished of Istanbul’s archaeological wonders, and some in the travel industry have voiced concern it could alienate potential visitors. The site drew 3.7 million visitors last year.
The drive to turn the sprawling building into a mosque, as demanded by pious Turks for decades, had become a matter of international concern. US Secretary of State Mike Pompeo urged Ankara to maintain the status of the site as a museum “as an example of its commitment to respect the faith traditions and diverse history” that make up modern Turkey.
“The United States views a change in the status of the Hagia Sophia as diminishing the legacy of this remarkable building and its unsurpassed ability-so rare in the modern world-to serve humanity as a much-needed bridge between those of differing faith traditions and cultures,” he said in a statement on Thursday.
Others worry the move symbolises a further deterioration of interfaith relations at a time of rising nationalism and identity politics. For secular Turks, changing the status of the Hagia Sophia is yet another example of rising Islamicisation under Mr Erdogan and his conservative Justice and Development Party (AKP), which has been the dominant political power in Turkey for nearly 18 years.
Hours before Mr Erdogan’s decree, Unesco issued a statement urging negotiations and warning that the “effective, inclusive and equitable participation of communities and other stakeholders concerned by the property is a necessary condition for the preservation of heritage and for the enhancement of its uniqueness and significance.”
Fatih municipality personnel place a safety barrier in front of the Hagia Sophia museum in Istanbul, (EPA)
In a speech last year, Mr Erdogan chided his own supporters for demanding the facility be converted, noting that Turkey’s faithful rarely manage to fill the country’s existing mosques.
But Turkey’s economy has deteriorated and polls have shown declining popularity for Mr Erdogan’s party, especially among the young. Mr Erdogan has increasingly sided with religious nationalists in the country’s culture wars.
Still, ranking Turkish officials have sought to reassure the world that allowing prayers at the facility will not alter its status.
“Turkey will still preserve the Christian icons there, just like our ancestors preserved all Christian values,” Ibrahim Kalin, a senior adviser to Mr Erdogan, told the official Anadolu News Agency on Thursday,.
He cited France’s Notre Dame Cathedral as a world-famous house of worship drawing both tourists and the pious.
“Opening up Hagia Sophia to worship doesn’t keep local or foreign tourists from visiting the site,” he said, “So a loss from the world’s heritage is not in question.”
The Hagia Sophia served as an Orthodox Christian cathedral for 1,000 years before it was turned into a mosque after the 15th Century Ottoman conquest of what was then the capital of the Byzantine Empire.