Controversy has raged over the impact of Edinburgh's expanded Christmas market before it has even opened this weekend.

Edinburgh World Heritage has broken its silence on the expanded use of the park to warn: “This must not set a precedent for future years.”

Its intervention has cast doubt over any repeat of the erection of scaffolding on top of the park to accommodate the market by claiming it “clearly disrupts this magnificent environment to a very great extent.”

The body, which is tasked with monitoring possible threats to Edinburgh’s world heritage status, said any future events in the gardens must be “more sensitive to the exceptional environment.”

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However Underbelly, the events company which has an £800,000 contract with the city council to produce the city’s winter festivals, has insisted that when the market opens it will an event “which both befits the environment and is sympathetic to it."

The company also pointed out that it had a contract in place with the council to run the market on the temporary structure it has erected in the gardens until 2021.

The charitable trust intervened the day after councillors pledged to explore alternative locations for the market, which attracts around 100,000 people a day during its six-week run in the gardens.

However the possibility of scrapping this year's market, which is due to open at the weekend, or moving it to a new location for the forthcoming festive season was ruled out.

Underbelly revealed last month that a record 163 stalls would be operated as part of a revamp to accommodate changes in the park since a major landscaping project was carried out as part of a £22 million project to improve access to the Scottish National Gallery.

However councillors say they were kept in the dark about an agreement to build a massive structure across Princes Street Gardens to try to protect the work done over the last year. It also emerged that work was allowed to begin in the gardens even though planning permission had not been secured from the authority.

A statement released today by Edinburgh World Heritage said: "The Waverley Valley forms a distinctive division between Edinburgh’s two exceptional urban phenomenon – the organic medieval Old Town and the planned Georgian New Town.

"The valley, which consists of Waverley Station, as well as Princes St. Gardens East and West, is an integral element of our World Heritage Site inscription, and central to our city’s identity.

"It is essential that any intervention within the valley – for example plans for Waverley Railway Station, or The Quaich Project redevelopment of the Ross Bandstand, all respect the integrity of the valley and preserve its role as what Scott called the ‘great arena’ between the Old and New Towns.

"Temporary interventions within the valley, such as the Christmas market in East Princes Street Gardens, must also respect the integrity of this unique space with its exceptional views and picturesque landscape and gardens.

"While temporary uses in our public spaces can play an important role in bringing many thousands of people into the city to enjoy what Edinburgh has to offer, this year’s proposal for the Christmas market, with its extended platform over much of the area in front of the Scottish National Gallery of Scotland, clearly disrupts this magnificent environment to a very great extent.

"While it may be too late to prevent the proposal going ahead this year, this must not set a precedent for future years.

"We call for any intervention in Princes Street Gardens to be more sensitive to the exceptional environment of Edinburgh Old and New Towns World Heritage Site.

"We support the decision of the city council to explore alternative locations for the Christmas market for next year."

Terry Levinthal, director of the Cockburn Association, the city's long-running heritage watchdog, said:

"We welcome EHW’s intervention and agree with the imperative need to respect the valley form as a fundamental design requirement."

Green Party councillor Alex Staniforth said: "This underlines the necessity of following the planning process.

"Had Underbelly applied for planning permission in the gardens, as they should have, then Edinburgh World Heritage would have had a chance to air these concerns much sooner.

"I’m glad Edinburgh World Heritage continue to pay close attention to our world heritage status - it is the unique beauty of this city that brings visitors and if we wreck that beauty for the sake of a ‘more more more’ attitude then Edinburgh tourism runs the risk of eating its own tail."

The Christmas market controversy has unfolded months after the launch of Citizen - a campaign to "defend" Edinburgh against over-tourism, gentrification, privatisation of public space and the impact of its ­festivals.

Spokesman Mike Small said: "Although this is a welcome move from Edinburgh World Heritage it does not go far enough. Wider questions about the role and conduct of Underbelly need to be explored. They seem to have a monopoly on public events in the city.

"Let’s start from the basis that Underbelly’s events have been a disaster and should be shut down immediately. They soak up public money, refuse to be transparent about their profits, cause environmental damage to public parks, and shut down and blight open spaces held in the common good.

"It’s entirely the city council’s fault that this has been allowed to happen but it needs shut down now. We need to completely re-think who and what these events are for. They are too often culturally hollow experiences which leave nothing for the city but disruption and chaos and a depleted public space."

However a spokeswoman for Underbelly said: "We appreciate the images of the build period have not been aesthetically pleasing but we can assure that the end result will be an event which both befits the environment and is sympathetic to it, as has been the case every time Underbelly has produced the event.

"Edinburgh’s Christmas is a hugely popular festival which brings many hundreds of thousands of people into the city centre for the benefit of the city, as well as underpinning the free and community events produced throughout the city and Edinburgh’s Hogmanay festival. We’re looking forward to sharing these events with the city and its visitors in the next few weeks.

"Underbelly has welcomed and will contribute to the council’s consultation on the winter festivals from 2022 onwards.