Swansea's Hendrefoilan House has been named the most endangered building in Wales after being nominated by the public.
The 19th century Welsh mansion was built by Welsh industrialist and MP Lewis Llewellyn Dillwyn.
It has been owned by Swansea University since the 1960s and was formerly used for accommodation and teaching.
But the building has fallen into disrepair, with water floods down the outside walls and damp on the inside, and has become a target for lead thefts.
It has now been included on the Victorian Society's list of the ten most endangered buildings in England and Wales.
Mr Costelloe criticised Swansea University, saying it had a responsibility to keep the building safe even if it has no current use for it.
He added: "An institution devoted to learning should have more respect for a building symbolic of Wales's industrial history and culture."
A spokesperson for Swansea University said:
The Victorian Society's top ten endangered buildings this year - in no order - are:
- Hendrefoilan House, Swansea
- Wingfield Station, Derbyshire
- Ipswich former County Hall
- Holborn Circus, London
- Agecroft Cemetery Chapel, Salford
- Butterley Spillway, Marsden, West Yorkshire
- Pumphouse, Langton Dock, Bootle, Merseyside
- Church of St Peter and St Paul, Birch, Essex
- Mechanics Institute, Swindon
- Waterloo Hotel, Smethwick, West Midlands
Last year the former YMCA building in Merthyr Tydfil made the list.