The Turkish president, who is now widely seen as an authoratarian leader with an Islamist intent, made the announcement after a court annulled the site's museum status.

The iconic structure and the pearl of Istanbul was built as an Orthodox Christian cathedral some 1500 years ago, but it was converted into a mosque during the Ottoman conquest in 1435.

However, for the past 80 years it was a museum and a glowing symbol of secularism which was once the bedrock of modern Turkey, created by Kemal Atatürk, a Turkish field marshal and revolutionary statesman who founded the Republic of Turkey, serving as its first President from 1923 until his death in 1938.

The decision comes amid a growing rise of the Islamists in Turkey who had been demanding that it be restored as a mosque, though Opposition leaders with secular credentials had been against the move.

A top court in Turkey ruled that turning it into a museum in 1935 by modern Turkey's secular architect Mustafa Kemal Ataturk was illegal, paving the way for incumbent president Recep Tayyip Erdogan to declare it as a mosque again and to open it for Muslim prayers.

Erdogan made the announcement an hour after the court ruled the conversion to museum in 1935 as illegal and scrapped its status.

Defending his decision, the president was earlier quoted as saying by BBC: "Like all our mosques, the doors of Hagia Sophia will be wide open to locals and foreigners, Muslims and non-Muslims."

Criticisms:

The decision earned criticisms with even UNESCO regretting the move.

Turkey’s Nobel prize-winning novelist Orhan Pamuk said as quoted in media: “To convert it back to a mosque is to say to the rest of the world unfortunately we are not secular anymore.”

The UN cultural agency (UNESCO) said that it "deeply regrets the decision" made "without prior discussion", having earlier in the day called on the State to abide by its “legal commitments and obligations” in accordance with its status as a museum, on the World Heritage List. 

Reacting to the decision, Director-General of UNESCO, Audrey Azoulay, described the building as "an architectural masterpiece, and a unique testimony to interactions between Europe and Asia over the centuries. Its status as a museum reflects the universal nature of its heritage and makes it a powerful symbol for dialogue."

UNESCO said in a statement issued before the decree, that it had written to the Government and called for “dialogue” with the authorities, “before taking any decision that might impact the universal value of the site.”

Hagia Sophia, is part of the “Historic Areas of Istanbul”, and officially inscribed on the World Heritage List as a museum.

“This inscription entails a number of legal commitments and obligations. Thus, a State must ensure that no modification is made to the outstanding universal value of the property inscribed on its territory”, said the UN’s Educational, Scientific, and Cultural Organization.

No modification ‘without prior notification’

“Any modification requires prior notification by the State concerned to UNESCO and then, if necessary, examination by the World Heritage Committee.”

“Outstanding universal value” is the basis for the decision to inscribe any property on the World Heritage List, the statement continued.

“The texts adopted by the Committee specify that the ‘Historic Areas of Istanbul’ is inscribed notably for ‘its unique integration of architectural masterpieces reflecting the meeting of Europe and Asia over several centuries’ and because ‘Hagia Sophia became a model for an entire family of churches and later mosques, and the mosaics of the palaces and churches of Constantinople, influenced both Eastern and Western art’”.

'Universal value'

The agency noted that in line with its designation, the Hagia Sophia “has a strong symbolic, historical and universal value.”

UNESCO called upon the Turkish authorities "to initiate dialogue without delay, in order to prevent any detrimental effect on the universal value of this exceptional heritage, the state of conservation of which will be examined by the World Heritage Committee at its next session."

“It is important to avoid any implementing measure, without prior discussion with UNESCO, that would affect physical access to the site, the structure of the buildings, the site’s moveable property, or the site’s management,” stressed Ernesto Ottone, UNESCO’s Assistant Director-General for Culture. Such measures could constitute breaches of the rules derived from the 1972 World Heritage Convention, the agency warned.

U.S. Senator from New Jersey Bob Menendez critcisied the movie and tweeted: "I strongly denounce #Turkey’s decision to convert Hagia Sophia into a mosque. I urge Erdogan to reverse course and restore Hagia Sophia’s remarkable legacy as a museum for people of all faiths and cultures to visit."

Economist Steve Hanke posted: " #BREAKING: The rigged #Turkish courts have cleared the way for @RTErdogan to turn the iconic #HagiaSophia museum into a mosque. With each passing day, #Istanbul looks more like #Tehran. The revolution just keeps rolling along."

Istanbul icon of beauty and wonderment

According to Turkey's official tourism website, Hagia Sophia is a remarkable achievement in the history of architecture. and a living proof of mankind's revolt against the laws of physics and it calls it a monument whose importance transcends borders.

It is one of UNESCO's World Cultural Heritage sites, attracting millions of visitors across the world with its majestic grandeur and beauty.

Dominating Istanbul's skyline, this quaint edifice has been expanded and restored several times throughout the centuries. Hagia Sophia has always been unique as an architectural piece from the first day it was built.

Hagia Sophia is not only renowned for its magnificent religious architecture but also for its superb exquisite Byzantine mosaics and fine examples of unique Ottoman calligraphy.

History of Hagia Sophia as described by Turkey in its official tourism website:

Church: Built by the order of Byzantine Emperor Justinian I in the 6th century, Hagia Sophia, the Church of Holy Spirit, includes pagan pillars taken from Ephesus and the Temple of Artemis and precious mosaics of Christianity and Islam. This great temple was dedicated to the Holy Spirit (Sophia) in the trinity doctrine of Christianity.

Hagia Sophia is also one of the largest cathedrals in the world having special significance for the Orthodox community.

Mosque: The church was converted into a mosque after the Ottoman Conquest of Istanbul in 1453. Ottomans buttressed the edifice with architectural elements displaying Ottoman character.

Mosque elements such as a mimbar, a mihrab, preacher's platform and wooden balustrades were added inside. Mahmut I had a library built here. The southern courtyard includes mausoleums of Murat III, Mehmet III, and Selim II. The original baptistery was converted into a mausoleum for Mustafa I and Sultan Ibrahim.

Museum: Hagia Sophia opened as a museum in 1935.

Postscript: It is a mosque again now with the decree on July 10, 2020