The Christian Aid warning comes as United Nations members and scientists meet to finalise a report on the impact of global temperature increases of 1.5C (2.7F) above pre-industrial levels and what is needed to prevent further warming.
Sea levels are expected to rise by more than 16in if global warming is not limited to 1.5C.
Christian Aid said climate change could act as a ‘threat multiplier’ to existing problems such as bad planning and subsidence caused by groundwater extraction and heavy buildings.
London is increasingly vulnerable as a result of sea level rises, with the city having to use its main flood defence, the Thames Barrier, more frequently, the report said.
When the barrier opened in 1984, it was predicted that it would be used two to three times a year, but it is now being used around six or seven times.
Other coastal or low-lying cities at risk include Houston, Shanghai, Bangkok, Manila, Jakarta, Lagos and Dhaka.
