An estimated 24,000 properties in London are at significant risk of flooding, a new report suggested.
Flood Risks in London, published by London Assembly's Environment Committee, also found that thousands more housing 140,000 Londoners are at high risk of surface water flooding with 230,000 thought to be at medium risk.
The Environment Agency estimates that plans under development could protect 10,000 of these homes, but 14,000 could be left at risk.
The high risk of river flooding was found to be in outer and west London and comes from tributaries of the Thames or the non-tidal' Thames.
The risk of flooding in the capital could also be increasing, the report states, as a result of the effects of climate change.
Drier summers and wetter winters could lead to more heavy rain and subsequently more flooding in London.
The committee also said that the record number of closures of the Thames Barrier in the winter of 2013/14 should be reviewed.
The committee called on the mayor to ensure London receives a "fair share" of national resources to invest in flood defences.