Officials in Edinburgh have recommended that councillors reject a plan to turn the disused Royal High School building into a hotel.
Developers have said the proposal would see the listed building on Calton Hill become a “world class” facility.
But a report presented to councillors said the plan would have a “significant adverse impact” on the site.
The development’s backers have insisted their proposal is the only realistic option for the site.
Also known as New Parliament House, the 19th Century building was once intended to house the Scottish Parliament.
A decision on the hotel plan will be made next week.
Planning officials said in their report: “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.”
The report said the plan does not comply with national guidelines on historic buildings, the city’s development plan or Edinburgh planning guidelines.
It includes partial demolition of the existing structure, and the building of additional accommodation wings for the hotel.
The report concludes: “Proposals for demolition do show that the luxury hotel would provide economic benefits to the city.
“However, the proposals fail to address the requirements of HESPS (Historic Environment Scotland Policy Statement 2016). Further, the demolition cannot be granted in isolation as test c) of HESPS is inextricably linked to the economic growth generated from redevelopment proposals.
“These proposals are considered unacceptable.”
Backers of the hotel plan have expressed their disappointment over the report.
They repeated their view that the proposal is the only one which guarantees the future of the original building.
Urbanist Hotels chairman David Orr said: “We need to be very clear about what is now at stake for the very future of the Old Royal High School.
“Our revised proposal is the result of 18 months intensive work and considerable investment in order to fulfil our contractual obligation with the Council. During this time we have taken time to listen to a wide range of stakeholders and heritage experts and taken their views on board.
“We now have a design solution which not only protects and promotes the magnificence of Hamilton’s centrepiece building but also has a viable and long-term investment plan in place to maintain it as part of the city’s living and breathing heritage.”
He added: “This is the only proposal that can realistically guarantee the future of the Hamilton building – both architecturally and financially.
“Without it, we risk another 50 years of disrepair and misuse, which would be catastrophic for both the building and the city.”