Claims arising from the Buncefield fuel depot blast could top £500m if local residents are successful in their bid to block the depot's owners Chevron-Texaco and Total Oil from rebuilding the plant in its current location.
If planning permission is refused, the claim, initially estimated to stand at £200m, could cost mutual insurer Oil Insurance Limited several hundred million more due to the cost of rebuilding the infrastructure in a different location from scratch.
Hemel Hempstead residents are believed to be backed by council leader Andrew Williams in opposing plans to re-build the facility.
David Way, Alexander Forbes' executive director, international division - energy, said that a refusal to grant planning permission would have "significant implications" for the final claims cost.
According to Way, domestic and commercial claims arising from the disaster were also likely to represent around £100m. Damage to the surrounding industrial estate accounted for £50m of the estimate, while business interruption claims could hit £20m.
Household and personal compensation claims were also likely to hit £10m, he said.
Meanwhile Nick Balcombe, chairman and chief operating officer of loss assessor Balcombe, suggested that the total insurance bill, including claims from residential and the nearby commercial estate, could be as high as £1bn.