Insurers could suffer from a surge in business interruption claims in London because of the rising water table, the Environment Agency has warned.
It is likely to affect thousands of businesses which store vital paperwork or computer equipment in basement offices.
"Insurers should be making businesses aware of the threat that the rising water table poses," said spokesman Dee Lockyer.
The rising water table is caused by the decline of heavy industry along the Thames, which had used vast amounts of water daily.
Flood damage is also a threat and it is a problem predicted to worsen over the next 30 years.
Document salvage and restoration company Document SOS found 70% of 50 London businesses surveyed were storing vital paper below the water level.
The Thames Barrier has been used to protect London from storm surges on 33 occasions since 1982, but is likely to be brought into use far more often in the future.
Flood levels are rising along the Thames which is predicted to rise by 8mm a year until 2030.