How the Royal High could look as a music school. Picture: Contributed

Scaled-back plans for the £75 million hotel, which will be decided on later this month, will cause “great and irreparable harm” to the former Royal High School on Calton Hill, according to Historic Environment Scotland.

It says it has “consistently” warned developers that turning the site into a hotel without is impossible without “unacceptable harm to the historic environment” due to the need to building two modern wings.

The body, which says the landmark is one of Scotland’s most important buildings, claims the current scheme will cause “significant” damage to the city’s world heritage site.

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It has suggests planners take into account the “less harmful” proposal to create a 300-capacity venue and new home for St Mary’s Music School as a way to “both safeguard the future of the building and preserve its historic setting.”

The stance reduces the prospects of a successful appeal if the hotel, which the Hong Kong-based Rosewood chain is due to run, is rejected again.

Dating back to 1829, the A-listed building designed by celebrated architect Thomas Hamilton has been lying largely empty since 1968.

The idea of turning it into a hotel won a council-run design contest in 2010, but the final plans sparked anger when it emerged they involved multi-storey extensions.

Councillors rejected the initial plans in 2015 but the number of hotel rooms has been lowered from 147 to 127, and the extensions dropping by up to 25 per cent.

In a 40-page submission, HES states: “The harm to the integrity, authenticity, character and significance of the building and its setting would be considerable.

“It would be impossible to view and fully appreciate Hamilton’s masterpiece, either by itself or in context, without the oversized extensions taking precedence, by their scale, siting and height. The proposals would, if implemented, diminish significantly the building’s current status and special interest.

“We’re not satisfied that this scheme represents the only option for the future use of the Royal High School. We consider that there is potential for less harmful approaches to its reuse.”

Adam Wilkinson, director of Edinburgh World Heritage, states: “Edinburgh is extraordinarily fortune in having, with the St Mary’s Music School proposal, a fully-funded scheme appropriate to the world heritage site and able to ensure the long-term future of the old Royal High School, protect its internationally significant setting and add to the quality of life of the citizens and visitors to Edinburgh.”