Major heritage bodies have told the city of Edinburgh that approving a plan to transform the old Royal High School into a hotel would destroy the city's credibility as a world heritage site.

In a joint letter to the Lord Provost of the capital, Frank Ross, four of Scotland’s leading heritage bodies - the Architectural Heritage Society of Scotland, the Cockburn Association, the Edinburgh World Heritage Trust and and the Scottish Civic Trust - have urged the city to respect the judgement of its own planning officers and refuse the plan.

Developers Urbanist Hotels and Duddingston House Properties have said the proposal would see the A-Listed Thomas Hamilton building on Calton Hill become a "world class" hotel facility.

However, a planning report presented to go before Councillors this week said the plan would have a "significant adverse impact" on the site.

A £35m rival plan to the £75m hotel proposal for new home for St Mary's Music School already has planning permission.

The joint letter - the first of its kind by the heritage bodies - says: "The pending decision on the planning application to build a hotel around the old Royal High School will resonate not just across the city.

"To approve this application would damage the city and the Council’s reputation for stewardship of its unique built environment, in Scotland, the UK, and internationally."

It adds: "The Council’s planning officers and the experts at Historic Environment Scotland have scrutinised the application and both recommend that it be rejected. T

"There is an overwhelming case to respect their judgement on this highly sensitive site...To disregard these policies would destroy Edinburgh’s credibility for stewardship of its World Heritage Site."

The bodies says that the plan would "irrevocably" damage the Grade A listed building and damage "forever" the city's Conservation Area and the capital's status as a World Heritage Site.

The bodies speak of the school's "profound" impact on the skyline of the city and add: "There is now a viable alternative in the St Mary’s Music School proposal, to which your Council has already granted planning permission and listed building consent, and which is generously funded by an Edinburgh philanthropist.

"The argument that the hotel is the only future for the building therefore falls."

Last week the council's planners said: "The proposed interventions to the listed building would have a significant adverse impact on the architectural integrity, composition and special character of one of the UK's finest listed buildings.

"Alternative consents, for the redevelopment of the building into an international music school, exist."