Up to 100,000 Cardiff householders could prejudice their insurance cover if they accept compensation for rising groundwater caused by the damming of Cardiff Bay.
The controversial Cardiff Bay barrage has created a massive artificial lake. As a result, thousands of Cardiff homes could suffer serious damage from rising groundwater.
The Cardiff Bay Development Corporation (CBDC) has begun a scheme to carry out remedial works on affected homes over the next 20 years. But as an alternative the CBDC is also offering householders compensation if their houses drop in value.
Andy Bellis, of Davies Chartered Loss Adjuster, warned policyholders that acceptance of compensation could leave them marooned if there is further groundwater damage.
"There is no doubt that policyholders can prejudice their own and ultimately their insurers' position," he said. "As yet, the actual impact of the Cardiff Bay barrage has yet to materialise but the future could be interesting."
Thousands of homes have now been designated as a Protected Property Area.
It includes the heavily-populated areas of Canton, Riverside, Grangetown, Adamsdown and parts of Splott and Roath, in addition to the industrial areas of Leckwith Moors, Butetown and Cardiff East moors.