INTERNATIONAL heritage experts including a Unesco adviser are to converge in the Scottish capital for a public meeting tonight amid concerns the revamp of a landmark building could threaten the city's World Heritage Site status.

Susan Denyer, secretary of the International Council on Monuments and Sites UK, which has a special role as adviser to the Unesco World Heritage Committee, will arrive from London to address the meeting as tension mounts ahead of the final unveiling of the design developers said will breathe new life into a disused A-listed architectural masterpiece.

The £55 million plans for the re-invention of the old Royal High School on Calton Hill as a hotel have been criticised for not taking account of architect Thomas Hamilton's ideas behind the structure.

The former boys' school was designed in a neo-classical Doric style by Hamilton and was opened in 1829 but has been unused since 1968 when the school was relocated.

The watchdog the Architectural Heritage Society of Scotland has called the meeting at St Andrew's and St George's Church in Edinburgh that will also include talks by Professor Alistair Rowan, former Principal Edinburgh College of Art, and Marcus Binney, chairman of Save Britain's Heritage, amid concerns the building's historic worth could be compromised.

Ahead of the meeting, Ms Denyer said: "Concerns over proposals for the old Royal High School have highlighted the outstanding historic urban landscape of Edinburgh and the need for its development to be driven by a dynamic vision, based on strong visual, cultural and intellectual engagement, so that new interventions support rather than subtract from its extraordinary assets."

Mr Rowan will address the meeting, saying: "Nestling against the wooded slopes of the Calton Hill, it forms an integral part of the World Heritage Site and is Scotland's iconic statement of the Neoclassical ideal.

"In Hamilton's great school building there is not one window in the central portico and none in the flanking colonnaded wings for his intention is to create a heroic composition, a temple to the value Scotsmen placed in education and to the belief that the security of the nation lay in the proper nurture of its sons.

"I do not believe that these ideals can now be brushed aside by punching windows through the walls merely to make the interior more pleasant for some elite visitors in a six star hotel.

"A monument is a monument and proposals for its conservation and future use must respect entirely those characteristics from which its monumental character derives."

Mr Binney said: "This magnificent example of Grecian architecture stands in a supremely beautiful position and nothing must be allowed to detract from it."

Historic Scotland has also raised concerns over the the plan to add windows to wings of the main building.

Andrew Martindale, of Historic Scotland, said: "The success of the building and its outstanding importance to the Greek Revival movement lies in Hamilton's adaptation of the windowless Greek temple to the modern use.

"The resultant architectural impact is enhanced by the flanking walls, pavilions and secondary elements, which combine to to reproduce one of the architectural high points of the 'Athens of the North'.

"While lacking the austere monumentality of the main facade, the secondary elevations are also meticulously detailed and very carefully thought out."

Over half of the 580 visitors to an exhibition at the site completed a feedback questionnaire which revealed that 79 per cent of them were generally in favour of the redevelopment proposals, with over 75 per cent agreeing that a world-class hotel would be an appropriate use for the well-known Edinburgh landmark.

David Orr, spokesman for project team Duddingston House Properties and the Urbanist Group, said: "We are in a formal process which recognises the views and opinions of a number of stakeholders and consultees.

"These responses continue to inform this process alongside our wider public events and consultation exercises.

"Our public exhibition earlier this month was very well attended and has provided much useful and positive feedback about the reuse of this important building.

"Our responses to all these views will be reflected in the public exhibition of the further developed proposals on March 5 and 6 from 10am -7pm at the old Royal High School."