The following factors are increasing the risk of sewer flooding:

• Climate change - leading to more intense storms
• Population growth - leading to increased housing development
• Paving over of green spaces - preventing natural drainage
• Disposal of fat, oil and grease - which harden in the sewers

Our sewers are designed to cope with the majority of storms, but occasionally storms are so heavy that they overwhelm the system.

London has a combined sewer system that takes in both sewage and rainfall - which means during a heavy storm, the flow in the sewer is much greater. London also has a high number of basement flats below street level, which are at greater risk of sewer flooding.

We believe sewer flooding is unacceptable, and our customers and stakeholders agree. We want to eliminate the flooding problems caused by a lack of capacity in local sewers.

Because we are committed to continuing to reduce the risk of sewer flooding to our customers, we are investing £346m on sewer flooding alleviation schemes to reduce the risk of flooding at 2,500 properties by 2015.