Plans for a new waste water treatment works at Sennen Cove near Land's End have been rejected unanimously by Cornwall County Council.
South West Water,which wants to upgrade the area's sewage facilities, says it is disappointed and will decide on a further course of action.
Local campaigners had argued it was not a suitable site for the works.
Proposals for another plant at nearby Porthcurno were recently turned down by Penwith Council.