The Environment Agency says that more than 107,000 properties in Lambeth and Southwark are at risk of an extreme 1 in 1,000 year flood - but only 195 homes are signed up to receive flood warnings.
New online flood forecasts and clearer warnings are part of improvements to the Environment Agency's flood warning service.
"The recent devastating events in Cornwall remind us of the dangers of flooding and how being prepared is crucial," says Thames regional director Howard Davidson.
The Environment Agency will now provide a daily, three day flood forecast for England and Wales on its website of flood risk from rivers, the sea and the potential for surface water flooding from intense rainfall.
The level of the tidal River Thames at Tower Pier can now be checked online.
Currently only 100 homes in Southwark and 95 homes in Lambeth are registered with the Flood Warnings Direct service.
The Environment Agency recently visited meetings of Borough & Bankside Community Council and Bermondsey Community Council to encourage SE1 residents to sign up. They also handed out emergency kits containing a torch, a radio, a disposable camera and water purification tablets.
Warnings issued to homes and businesses by phone and text message will now include more localised information about potential flood risk with clearer advice on actions to take to help safeguard lives and property.
The existing 'flood watch' – the lowest level of warning – is replaced by 'flood alert', and icons have been redesigned to make them clearer and easier to understand. 'Flood warning' means that flooding is expected, urging people to take action to prepare.
A 'severe flood warning' will be issued when flooding poses a danger to life.
More information on the new flood warning codes can be found on the Environment Agency website. www.environment-agency.gov.uk/fwd
Those wishing to sign up to the free flood warning system can call 0845 988 1188 or visit www.environment-agency.gov.uk/flood