A striking combination of glass, curves and straight lines -- the Tsentrosoyuz building in downtown Moscow stands as a monument to modernist architecture and the genius of its famed designer Le Corbusier.
Completed in 1936 to house the Soviet Union's ministry of light industry, the inventive office block was the first major public commission received by the legendary Swiss-French architect.
It is a symbol of the experimentalism of the early years of Communist rule -- but conservationists are now worried that the historical building could be overshadowed by a 58-metre (191-feet) tall business centre, being mooted for right next door.
For the time being, there is no question of Le Corbusier's creation -- renovated in 2013 -- being touched.
It is classified as a protected historic monument by the authorities and the current inhabitants -- state statistics agency Rosstat -- have no intention of moving out.
But the potential destruction of two neighbouring late 19th century residential buildings to make way for the new business centre has set some preservationists on edge.
"If this project is realised it will change in a very regrettable way the immediate environment of the Tsentrosoyuz," Antoine Picon, the president of the Fondation Le Corbusier, wrote in a letter sent in January to Russia's culture minister and the mayor of Moscow.
"The design took into account the make-up of the buildings that existed at the time," he wrote.
Picon told AFP that he was still to receive a reply to his letter.
The Moscow building applied some of Le Corbusier's architectural principles, including a curtain-wall facade and flat roof, and was made of concrete and red tuff stone, a type of volcanic rock.
One of the fathers of modern architecture, Le Corbusier's ideas about utilitarian concrete buildings have altered the face of cities across the planet and have had an equally profound influence on urban planning.
According to preservation group, Arkhnadzor, the buildings slated to make way for the business centre are owned by Russia's defence ministry, but a private investor, Fin.Kom, now holds the rights to develop the 15,000-square-metre (161,400-square-feet) site.
When contacted by AFP, Fin.Kom did not respond.
Moscow's cultural heritage authorities told AFP that they have not yet "received or agreed to" any application for work at the site.
But they confirmed that the buildings were not classified for preservation and could, in principle, be torn down.
"We do not currently have knowledge of any threats to the Tsentrosoyuz," they added.
But in a sign that preparations for the development could be speeding up, a group of state-approved planning experts last year issued a favourable ruling for the developers.
They insisted that, given the "enormous changes that have taken place over the past quarter-century" in the area, the two residential buildings are now "obsolete".
City officials must still approve the experts' findings.
But in December, city hall published draft development plans that included replacing two old residential blocks with a 58-metre high building.
For those fighting to protect Moscow's architectural heritage, these moves are a major blow.
Some 17 Le Corbusier projects around the world were added to UNESCO's World Heritage List in 2016 and, while Tsentrosoyuz is not among them, it is viewed as a potential future candidate.
Experts say erecting the business centre next door would likely dash those hopes.
More generally, as the Moscow skyline has changed dramatically in recent years, conservationists complain that the project is symptomatic of official disregard for protecting the historic gems studding the city.
"To put a construction that will be twice as big next to such a building... that means reshaping the whole district and disfiguring its appearance even more," said Konstantin Mikhailov, of Arkhnadzor, which works to preserve Russian heritage.
"Moscow is a place where there is a concentration of riches and where there exists a lot of pressure from investors," he told AFP.
"Authorities in the city can't always resist this pressure and sometimes they even encourage it themselves."