Converting Istanbul's Byzantine-era cathedral Hagia Sophia back into a mosque will allow Turkish President Recep Tayyip Erdogan to expand his Islamic and nationalist base and divide the opposition, experts say.

But the move will heighten tensions between the West and Turkey's veteran leaderErdogan, who has been grappling with an economic crisis and regional conflicts in several nearby countries, they say.

Critics accuse Erdogan of undermining the secular credentials laid down by Kemal Ataturk - the founder of modern Turkey.  

Founded 1,500 years ago as a cathedral, the Ottomans made Hagia Sophia a mosque but it was turned into a museum in 1934 and is a UNESCO World Heritage Site.

Erdogan, who has in the past repeatedly called for the stunning building to be renamed as a mosque, signed a presidential decree on Friday, handing over Hagia Sophia to Turkey's religious affairs directorate for reopening to Muslim worship. 

Despite its secular status as a museum, Islamic rituals have been performed in the complex on several occasions, including lavish celebrations to mark Istanbul's conquest by the Ottomans, since Erdogan's Justice and Development Party (AKP) came to power in 2002.

"Hagia Sophia's museum status was seen by many who support Erdogan's government as a dispossession," said Jean Marcou, associate researcher at the French Institute for Anatolian Studies. 

"Erdogan intends to reaffirm Turkey's power and Muslim identity with this approach seen in many ways national as much as religious," he said.

Converted for the first time into a mosque after the conquest of Constantinople by the Ottomans in 1453, the Hagia Sophia is one of Istanbul's most visited tourist attractions.

Ozgur Unluhisarcikli, Ankara director of the German Marshall Fund, said the move would win hearts and minds as the majority of the Turkish public "would favour such a decision for religious or nationalist sentiments".

"This is a debate president Erdogan cannot lose and the opposition cannot win. As a matter of fact, this issue also has the potential to disunite the opposition parties," he told AFP.

'Scoring points'

The traditional voters of the secular Republican People's Party (CHP) are not too interested in the Ottoman past, said Unluhisarcikli.

"(But) they are very sovereignty conscious and some of them could support the decision just because others are telling Turkey it cannot do this," he said.

Turkey's fellow NATO members - the US and Greece - as well as Russia, have warned Ankara against reopening the Hagia Sophia to Muslim worship. 

"The decision is intended to score points with Erdogan's pious and nationalist constituents," said Anthony Skinner of the risk assessment firm Verisk Maplecroft. 

"Hagia Sophia is arguably the most conspicuous symbol of Turkey's Ottoman past – one which Erdogan is leveraging to strengthen his base while snubbing domestic and foreign rivals," he said. 

The court ruling on Hagia Sophia's status comes amid escalating tensions between Turkey and the European Union over Ankara's controversial drilling strategy in the eastern Mediterranean and its involvement in the Libyan conflict. 

Turkey has sent drones and Syrian fighters to support the UN-backed government in Tripoli, angering France which is suspected of backing a Libyan renegade general.

Hagia Sophia's transformation into a mosque in defiance of the West "is consistent with Turkey's muscular foreign policy", said Skinner. 

"It is consistent with the government's projection in the east Mediterranean and Libya."

Marcou said altering Hagia Sophia's status would "move Turkey even further away from its Western allies, affect Greek-Turkish relations and likely hamper Russian-Turkish ties. 

"Symbolically, such a decision would appear as the culminating point for Turkey which has systematically gone on the offensive in all theatres of regional conflicts: Syria, Iraq, Libya and eastern Mediterranean," he said.