ISTANBUL: Turkish authorities have sealed off a luxury spa run by a five-star Istanbul hotel that was set up by illegally modifying a 1,500-year-old cistern, officials said on Monday.

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 Fatihmunicipality in line with a decision by its preservation committee,` a municipal official said.

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 Turkiye. A Dorak spokeswoman said she was unable to comment as `a legal process was ongoing`. A spa employee also refused to comment.-AFP