Backed by the philanthropic Dunard Fund, the Trust has made a formal legal offer of £1.5 million to buy the iconic buildings and will pursue plans to restore the site for St Mary’s Music School.

The Trust now intends to submit a detailed and fully funded application to the City of Edinburgh Council as soon as possible.

The vision will provide an alternative to Hoskins Architects’ £75m plans to turn the school into a hotel.

William Gray Muir, the chairman of the Trust, confirmed that a bid to purchase the Old Royal High School exceeds the value currently put on the buildings by the City of Edinburgh Council. This is a small part of a substantial funding pledge which has been made to the Trust to underwrite the cost of conservation and conversion that respects the buildings’ original character, historic significance and location to secure their long-term future.

The project will represent one of the single largest philanthropic arts gifts in modern Scottish history.

He said: “The Old Royal High School buildings are crucial to the character of Edinburgh and part of the architectural heritage that attracts people to this wonderful city. The Trust was set up specifically to conserve and protect this masterpiece for the long-term and what better way than to restore it to its original purpose as a school?

“St Mary’s Music School needs a new home and if we are successful, not only will this move ensure the School will have more teaching and performance space but as a new venue and destination for talented musicians it will also create opportunity to add to Edinburgh’s cultural economy.”

Responses received as a result of a pre-application public consultation process so far suggest a very high degree of public and community support for the RHSPT’s proposal.

William Gray Muir added: “We are delighted to have persuaded Richard Murphy to take on this project. Richard’s architectural practice is vastly experienced in both arts and education buildings and is the ideal partner for such an important project.

“The community response that our proposals have so far received has been phenomenal, with almost 100 per cent of people surveyed at two public events telling us that they support the Trust’s vision for the old Royal High School.  We will continue to consult as part of this process and plan to present an update on our designs within weeks.”

Richard Murphy Architects and Simpson & Brown have expressed delight to have been appointed.

Richard Murphy said: “This is a wonderful site, an exceptionally important building and a delightful client and brief; what architect could not relish such a prospect? We look forward to working closely with the Trust, the School and the rest of the design team.”

John Sanders, partner at Simpson & Brown, added:  “We have enjoyed working together with Richard Murphy on previous projects and the Royal High School project is a wonderful opportunity to combine creativity with scholarship.”

The council said it is due to decide on the hotel plans by Duddingston House Properties in December and it cannot consider the rival offer at the moment.

A spokeswoman for Edinburgh City Council said: “Old Royal High School is not on the market as the council has a legal agreement with Duddingston House Properties to lease it. The site is now subject to a planning application for a hotel, which has been submitted to the council and will be considered in due course.”