Heritage watchdogs claim the £70 million project is too large and will obscure views of Edinburgh Castle - a designated World Heritage Site.
In a submission to Edinburgh City Council, Historic Scotland inspector Dr Robin Evetts panned the scheme: 'The overall height may in fact compete with the prominence of the conference centre, as well as detract from the skyline. We therefore suggest that the overall height of the building should be reviewed.'
Architecture and Design Scotland and conservation body the Cockburn Society have also attacked the scheme, claiming it dilutes the aesthetics of the EICC - originally designed by Terry Farrell.
A year ago, Farrell was accused of breaching procurement rules after it was revealed his involvement with EICC developer CALA-Morrison conflicted with his role as Edinburgh urban design czar (AJ 1.7.04).
Farrell's scheme will effectively double EICC's capacity. It will be two storeys higher than the existing conference centre and feature a 1,600-seater exhibition hall together with 15,000m 2of retail and office space.
The extension is considered crucial to Edinburgh's status as an exhibition and conference destination.
Terry Farrell & Partners was unavailable for comment.