Saint Petersburg, the imperial capital of Russia famed for its elegance and beauty, risks losing status as a world heritage site under plans by a Scottish company to build the highest tower in Europe there.

RMJM, Edinburgh-based co-architects of the contro-versial Holyrood building, have designed the £1bn-plus, 396m Okhta Tower as headquarters for state-controlled Gazprom - one of the world's largest energy companies.

The proposals have promp-ted an outcry from heritage and conservation groups that it would ruin St Petersburg's historic skyline.

Independent watchdog the World Monuments Fund (WMF) has included the western Russian city in its 2008 World Monuments Watch List of 100 Most Endangered Sites as a result of the plans.

The St Petersburg Union of Architects said the 77-storey tower would be an "architectural crime" while other critics dubbed the massive project, initially known as the Gazprom Tower, "Gazilla".

Unesco now appears to be on a collision course with RMJM and Gazprom after talks in Moscow last week failed to produce a resolution.

The heritage body said planning permission could still be refused and called for a new report on the impact on the skyline from the planning authorities by February. Work on the the five-sided tower that twists as it rises is due to start in the middle of next year and be completed in 2016.

Dr Francesco Bandarin, director of Unesco World Heritage Centre, said: "Since we received news of this project, Unesco has expressed its reservations as we believe that the architectural solutions proposed are not in line with the nature of this important heritage site and may affect the values inscribed in the World Heritage List."

He said the World Heritage Committee in July asked for a "redefinition of the design, in order to reduce its visual impacts", but while RMJM said after last week's meeting that it believed it had addressed the issue and "disproved" concerns about height by providing new images of how it would look, Dr Bandarin remained unsatisfied.

He said: "I really apprec-iated the professionality of RMJM, I think this firm is a world-class architectural design group.

"Most of the proposed project responds to very high design standards and is fully adequate as an urban rehabilitation proposal.

"However, I maintained Unesco's objections on the design of the tower, that I found inappropriate in scale and height, and that will certainly affect the urban landscape of the city.

"I was informed by Gazprom that the solution proposed is not yet final and that even the formal approval of the project by the city last June has left undecided the issue of the tower.

"I encouraged Gazprom to look for solutions that could limit the impacts of the design on the urban landscape and offered Unesco's collaboration in this endeavour."

A spokesman for the WMF, a US-based charity, said: "Gazprom, Russia's largest oil company, plans to build a tower which will dramatically change the skyline of historic St Petersburg.

"If it goes forward, it could establish a dangerous precedent of inappropriate siting of tower blocks in historic towns.

"Furthermore, the project could jeopardise the historic city centre's Unesco world heritage status."

The tower is being funded by the local government, or City Administration, paying 24.9bn roubles (£500m), and Gazprom subsidiary Gazprom Neft providing 30.6bn roubles (£700m) for the construction of the centre - a total of up to £1.2bn.

RMJM said the attraction for the city includes tackling its "industrial legacy" and benefits for residents: office space for businesses and commercial units; public green spaces; public library; concert hall; museum; an ice rink; and public viewing gallery from the top of the tower.

Tony Kettle, RMJM's chief architect, said: "I think the quality of the tower's design and its limited impact is critical here and we firmly believe our design truly works for the city.

"The historical part of the city is based on a tradition of vertical dominants against the main horizontal skyline. The existing vertical dominants include the spire of the Peter and Paul Cathedral, the Admiralty, St Isaac's Cathedral, and the TV tower - the tallest structure in the city at 316m high same height as the Eiffel tower.

"St Petersburg is not the place to create a collection of towers like Manhattan or Paris' La Defense.

"We have created something quite unique and timeless, a beautiful landmark for the city which will also set new standards for energy conservation and sustainability.

"Gazprom is one of the world's most important energy companies and it is fitting that in a city of spires, this new spire should sym- bolise the importance of energy."

Boris Vishnevsky, a member of the Yabloko party which has strongly opposed the development, said the construction "will make the city look ugly forever".

Unesco, United Nations Educational Scientific and Cultural Organisation, said that it would maintain contact with the Russian planning authorities and Gazprom.

Much-loved Venice of the north'

  • Much-loved by architectural historians and its inhabitants alike, St Petersburg is often called the "Venice of the north," with its numerous canals and more than 400 bridges, according to the World Monuments Fund.
  • A vast urban project begun under Peter the Great in 1703, St Petersburg was also an experimental playground for the leading European architects of the eighteenth and nineteenth centuries.
  • Its architecture is a blend of baroque and neoclassical styles as seen on the Admiralty, the Winter Palace and the Marble Palace buildings.
  • Characterised by its low city skyline and historic integrity, the centre of St Petersburg was inscribed on the Unesco World Heritage List in 1990.