Unesco advisers seek probe into Edinburgh World Heritage site after controversial plans including former Royal High school hotel on Calton Hill

The UK national committee of the International Council on Monuments and Sites (ICOMOS), which has a special role as official adviser to Unesco, criticised the planning culture in the city.

The advisers said: "Evidence of the potentially negative impacts of cumulative new developments is becoming increasingly apparent."

Their report stated: “Whilst no objective study exists of the impacts or likely impacts on Outstanding Universal Value (which could affect heritage status), the widespread expressions of concern which have been made known to ICOMOS convince us that a serious investigation into the cause is merited.”

The report called for greater scrutiny over plans at an earlier stage, and backed the overhaul of the long-term heritage blueprint that would help maintain the prestigious status.

The advisers said: "Given the current trend for something like 10 to 15 significant developments in the central areas of the city each year, it would appear timely to debate how the impact of around 100 to 150 new developments in the city over the next 10 years might have a beneficial impact in cultural as well as social and economic terms."

ICOMOS continued: "The proposed revision of this (World Heritage management) plan should be an opportunity to re-frame the document to make it more accessible and user friendly, and more conceptual.

"It could deal with governance and decision making processes and perhaps be less like a structured ‘plan’."

It came as controversial plans to restore the former Royal High School in Edinburgh as a world-class hotel were said to have the support of 93 per cent of Edinburgh residents polled in a survey commissioned by the developer.

It is claimed one of the largest polling exercises carried out in the city in recent times involved almost 5,000 people across 12 districts who were interviewed on the streets and asked their opinion on the £75 million desvelopment to revitalise the A-Listed structure. The proposal will go before planners in December.

The survey found 78 per cent interviewed believe the proposal to save the Thomas Hamilton neoclassical masterpiece will have either a positive or no effect on the city’s World Heritage status.

Taco van Heusden, of Royal High developer Urbanist Hotels, said: "The measured report from ICOMOS and its call for a more sophisticated dialogue between stakeholders around heritage planning applications is to be welcomed.

“It is also clear from the ICOMOS report that despite recent scare-mongering Edinburgh’s World Heritage status is not under threat, which is in line with public sentiment on the matter.”

Adam Wilkinson, director of Edinburgh World Heritage, said: “We particularly welcome the acknowledgement of the tremendous efforts to conserve the city over the last 45 years.

"The suggestions for the review of the management plan and moves to the UNESCO Historic Urban Landscape approach are sensible, taking a holistic and integrated approach to the management of historic cities.

"We have been working to these principles for many years, and will seek their formal adoption in the forthcoming review of the management plan. Wider suggestions relating to the long term approach needed in looking after a sustainable historic city centre are also welcome.”

Marion Williams of the heritage body the Cockburn Association said: "I endorse everything it (the report) says and welcome positive moves to up our game in terms of managing the world heritage site for the future."

Ian Perry, Edinburgh city council planning convener, said: “As a major, European capital it is inevitable that new developments are attracted to the centre, and this is central to our reputation as a thriving, progressive city.

"However, we are committed to managing this process very carefully, striking a fine balance between economic development and the maintenance of our historic environment.”