Duddingston House Properties and Urbanist Hotels want to redevelop the A-listed building on Calton Hill, which has lain empty for almost 50 years, into a 127-bedroom hotel.

Their proposal was unanimously rejected by the City of Edinburgh Council last month amid concerns it would harm the historic nature of the site and the landscape of the city.

Lewis Ritchie, the council’s planning convener, described the designs as “abhorrent” and “ugly,” and said they were “almost universally reviled” by anyone who had seen them.

However, David Orr, the chairman of Urbanist Hotels, told The Scotsman it remained determined to find a future for the old Royal High School.

He said: “In line with our contractual agreement with City of Edinburgh Council, we remain wholly committed to delivering an outstanding scheme for the old Royal High School, reviving a building which has been allowed to slip into a state of disrepair and neglect for nearly 50 years.

“We fully recognise the importance of Hamilton’s Old Royal High School Building on a national level and our revised proposals guarantee the future of Hamilton’s masterpiece, both architecturally and financially.

“As such, we have advised the planning and environmental appeals division of the Scottish Government that it is our intention to appeal.”

Lawyers for the developers have already written to the Scottish Government department advising them that an appeal will be forthcoming.

Ann Faulds, a partner at law firm CMS, e-mailed officials last Thursday to advise them that an appeal would be lodged “as soon as practicable”, once supporting documents had been gathered and a formal decision notice was received.

The Hoskins Architects-designed project was scaled back in February after an initial bid was rejected by a single vote in December 2015 after councillors decided the original plans, which included two six-storey extensions, would damage the building’s Calton Hill setting.

The revised project proposed 127 rooms, 20 fewer than originally envisaged.

A spokesman for Mr Orr confirmed Duddingston and Urbanist intend to pursue that appeal alongside the new one.

A rival vision by Richard Murphy Architects and Simpson & Brown to turn the property into a music school has been approved but cannot proceed until an agreement between the council and Urbanist Group expires.