The Scottish capital's heritage watchdog has said architects should go back to the drawing board over plans to turn an historic landmark into a luxury hotel.

The Cockburn Association delivered a scathing critique of the latest proposals to transform Thomas Hamilton's neo-classical masterpiece the old Royal High School into a £55 million hotel which have attracted criticism from bodies including Historic Scotland.

Experts raised concerns over aspects of the plan including introducing widows and the impact the revamp could potentially have on the city's prized Unesco World Heritage Site status.

Cockburn Association director Marion Williams said that "an alternative use which is compatible with this unique A listed building within Edinburgh's World Heritage Site should be brought forward".

The developer Duddingston House Properties and the Urbanist Group said it is evaluating the views of the public and heritage bodies as it prepares to lodge detailed plans.

Ms Williams said: "The proposed windows in the portico appear to be fundamental to the concept of the hotel foyer and a spectacular arrivals space.

"The great achievement of Hamilton was to create a Greek temple composition without any windows, something which his contemporaries were very rarely able to do.

"This is the essence of the Greek revival."

She added: "In summary, there is a fundamental discord in the adaptation of a building with no windows to the view into a hotel, it will also result in significant modification to the south facade of the school, the very thing that the whole scheme has been devised to protect.

"The High School building is protected as an internationally important example of architecture.

"The original brief for the developer was initially untested."

She added the tabled proposals "undermine the building's important relationship with its context".

"The volume of accommodation which is stated to be required indicates that the use of the building and site for a hotel is not appropriate.

" It is important that the old Royal High School and the site are protected for current and future generations of Edinburgh's citizens and visitors."

The proposals for the former boys' school, which opened in 1829 but unused since 1968 when the school was relocated, are still being formed and a public exhibition is being held there next week.

David Orr of the Urbanist Group said: "We are currently in a consultation process and are engaging with the public by way of exhibitions and online feedback.

"It is the latest stage in a public process initiated by the City of Edinburgh Council after many years of suggested projects failing to attract funding.

"We have secured funding and following ratification in April 2014 and in line with our obligations, we are bringing forward our proposals to ensure the restoration of this important building, giving it a sustainable future that delivers wider benefits for the Calton Hill area and the city of Edinburgh."

He added: "There has been a wide range of views expressed from the questionnaires filled in from the first exhibition and we are going to ensure that this feedback is reflected in the next exhibition, to be held on March 5 and 6 at the old Royal High School.

"Whilst the views expressed so far are overwhelmingly supportive, we recognise that less positive views have also been expressed and we will faithfully ensure transparency on feedback before the planning application is submitted.

"We want to ensure the public has every chance to comment on our proposals to sensitively restore Hamilton's masterpiece.

"As with the first exhibitions we will be there over the two days to explain our proposals as they have developed."