An Bord Pleanala has refused a planning application from SIPTU to redevelop Liberty Hall in Dublin.
The authority has said planning permission was refused because the proposed new 23-storey building would be "unacceptably dominant in the city and would be visually intrusive in the streetscape and riverscape".
It further added that the development could "seriously detract from the setting and character of the Custom House, and would intrude on the O'Connell Street and Grafton Street Architectural Conservation Areas".
The proposed tower would have been 35% higher than Liberty Hall.
It was SIPTU's second proposal for the site, the union said it was in need of serious upgrading.
Dublin City Council had previously granted permission for the development, though it had included 19 conditions, including a realignment of some of the upper levels of the building.
Irish Life Assurance plc, The Heritage Council and others had appealed the decision to An Bord Pleanála and this appeal has now been upheld.