Insurers are failing to differentiate between high- and low-risk composite panels, Marsh has warned.
Marsh's advisory note on panels said acceptance of Loss Prevention Council-approved panels was not universal.
"There's much confusion about definitions of composites and the fire risks they pose," the note said.
"A number of insurers now take a hard-line approach for risks where there is evidence of substantial use of composite panels in the construction of buildings.
"They tend to view composite panels generally as a bad feature [and] to date, most have not differentiated between internal panels and cladding panels."
The broker said some property owners had been completely unable to obtain cover for buildings.
"Consequently, some occupiers now operate without property insurance, which can have serious implications for lenders and financial backers," the advisory note said.