Belfast City Council has confirmed that it is undertaking a live enforcement investigation in relation to "unauthorised" work at Signature Living's George Best hotel.
Signature Living is currently developing the 63-bedroom hotel in Belfast, named after the late Northern Ireland footballer.
It was originally due to open in June this year but its launch has been delayed as planning consultations continue.
Now the Department for Communities' Historic Environment Division (HED) is understood to have written to the Council's planning authorities, raising concerns over "unauthorised works" at the development, according to the Belfast Telegraph.
The news prompted the not-for-profit group Ulster Architectural Heritage (UAH) to call for an immediate stop notice to be issued on the work at the listed former Scottish Mutual Building in Donegall Square.
UAH chief executive Nikki McVeigh said she was "extremely concerned" about the allegations of unauthorised works at the building.
She said: "UAH has repeatedly expressed concerns to responsible authorities about the works to this building by current and previous owners. With the knowledge that the building has very important interior features, including elements of tiles, joinery, fireplaces, plasterwork and more, UAH has for some time been concerned about the content of promotional material for this project which illustrates generic, new build hotel interiors, and not those we understand to be original, appropriate or sympathetic to this important and prominent listed building.
"We call for an immediate stop notice to be issued until this urgent matter is resolved."
A Belfast City Council spokesman said: "Belfast City Council can confirm that a live enforcement investigation is currently open in respect of this building. Due to the live nature of this investigation, we are not in a position to make any further comment."
A spokesperson for Signature Living said: "We are working proactively with the HED/BCC in relation to the progress of the redevelopment of the George Best hotel, including a meeting with all parties concerned.
"Information in relation to those queries raised is now with all necessary parties to address. We are continuing to work with them to obtain the necessary consents.
"With regards to the live enforcement investigation, I can confirm this relates to a complaint made by a neighbouring business. Once a complaint is lodged, this has to be followed up by an investigation. This is natural due process and which we have referred to in a previous statement earlier this month.
"Lawrence and his Belfast team look forward to welcoming their first guests in early 2019 to the George Best hotel ."