Vienna's historic city centre has been put on UNESCO's danger list over plans for a high-rise development by Brazilian architect Isay Weinfeld, which is to include luxury apartments, a hotel and skating rink.
UNESCO's List of World Heritage in Danger highlights areas of cultural significance at risk of ruin through redevelopment.
The United Nations Educational Scientific and Cultural Organization added Vienna to the watchlist earlier this year after plans for the high-rise development within the confines of the cultural heritage site were approved.
A 75-metre tower included in the original proposal has been lowered to 66.3-metres to appease objections to the height of the development.
But the World Heritage Committee, which is in charge of drawing up the list of significant sites, ruled the plans for the Heumarkt area threaten the characteristics that saw the city placed on the World Heritage List in 2001.
"The List of World Heritage in Danger is not meant to be a step towards delisting but a way to raise awareness of threats and mobilise support for the conservation of sites that are in danger," a UNESCO spokesperson told Dezeen.
"I do not feel comfortable predicting the future [of Vienna]."
Delisting is an extreme event that has only occurred twice in the 40-year history of the World Heritage List, when sites have been irreparably damaged. Putting Vienna on the danger list is a warning shot for the city's planning department.
"The committee, and UNESCO, which serves as its secretariat, does not delist sites lightly," they added.
"The whole idea of the World Heritage Convention is to promote conservation and the sharing of expertise and experience in this area. This means that we are never happy to give up on a site recognised to be [of] outstanding universal value."
Vienna was awarded world heritage status for its grand baroque architecture and its history as a European centre of music.
Overseen by property investors WertInvest, the Heumarkt project encompasses the Hotel Intercontinental Vienna, the art nouveau Weiner Konzerthaus, and the Viennese Skating Club.
Wienfeld and Austrian studio Sebastian Murr won a competition to design the project back in 2014. The scheme involves remodelling and heightening an existing hotel, and creating an indoor skating rink, sports facilities and conference centre.
UNESCO had previously set the height limit for any new buildings in the city centre at 43-metres. However, Vienna's city government didn't set a height restriction for the competition, leaving it at the discretion of the architect and the developer.
In an interview at the time Weinfeld stressed that the architecture would be "subtle" and "discrete" but opponents of the project are particularly unhappy about the height of the tower.
In March 2017 revised plans were submitted for approval, lowering the height of the tower by three storeys and reducing the space for the apartments by 24 per cent. The plans also saw the hotel repositioned to be slightly further back from the Konzerthaus.
Vienna city council approved the project in June 2017, and construction is expected to begin at the start of 2020.
"The project will be more than worthy of the confidence it has been shown in it and will contribute to consolidating Vienna's position as the most liveable city in the world," WertInvest's managing director Daniela Enzi said in a statement at the time.
Enzi added that the process of reconciling the public and private interests of the project had been "a very rocky one, but ultimately also a very fruitful".
The alterations were not enough to satisfy the World Heritage Committee, which voted to put the Austrian capital on the List of World Heritage in Danger at their annual meeting in July.
The committee also urged Vienna's state party, currently a coalition between the Green Party and the Social Democrats, to halt all high-rise project approvals until stricter planning controls around building height, volume and density are introduced.
UNESCO also requested that all proposals be subjected to a Heritage Impact Assessment prior to approval, including the plans for the hotel, and the area around the Konzerthaus and skating club.
Vienna's city government has until February 1 2018 to submit a plan to implement these recommendations in order to have the city removed from the danger list.
In the UK, Liverpool's UNESCO World Heritage docklands area has also been put on the danger list this year due to plans for a pair of high-rise residential blocks opposite the River Mersey.
The £5.5 billion Liverpool Waters development, the Plaza 1821 and Hive City Docks towers by Hodder and Partners and Brock Carmichael Architects were approved despite being far taller than the other buildings in the area.
Renderings courtesy of Nightnurse.