The Navitas Spa Sarnic is operated by the Hagia Sofia Mansions Istanbul hotel, which is part of Hilton's luxury Curio Collection that currently offers top-price suites for around $400 per night.

The hotel, just a short walk from the Hagia Sofia, a UNESCO World Heritage site, modified the ancient structure to include a jacuzzi-style swimming pool, sauna and massage rooms and a Turkish hammam.

The revelations about the work sparked an online backlash on social media and eventually local officials took action.

"The swimming and spa centre near the Hagia Sophia mosque... was sealed off by teams from the Fatih municipality in line with a decision by its preservation committee," a municipal official told AFP.

The culture and tourism ministry also said it had lodged a complaint in 2023 about "unauthorised practices" concerning the spa.

The hotel is in the Sultanahmet neighbourhood, known for the Grand Bazaar, the Blue Mosque with its iconic minarets and the Hagia Sophia.

"Our spa offers a Turkish Bath and a restored 1,500-year-old cistern," the hotel website says, showing photos of a tastefully-lit brick-lined underground pool with bubbling turquoise waters.

The municipality said the cistern was owned by Dorak, a holding company with tourism assets across Turkey.

Contacted by AFP, a Dorak spokeswoman said she was unable to comment as "a legal process was ongoing".