Development plans for East London and the Thames Gateway are putting the city at risk of serious flooding, a report has warned.
The Thames Barrier, London's main flood defence, is one of 2,400
The London Assembly study said the city must learn from New Orleans, and urged action to be taken immediately to ensure London is adequately protected.
The report also said confusion over who runs London's flood defence system was putting the city in danger.
But the Environment Agency said the flood defences are in good order.
According to the Assembly's Environment Committee report the area at risk in the Thames Estuary is home to 1.25m residents and 400,000 properties.
"We're extremely concerned about development plans for East London, and the building of thousands of new homes in Thames Gateway. These plans are simply not taking the flood risk issue seriously enough," committee chairman Darren Johnson said.
"Much of the development area is on the flood plain, which will put London at greater risk."
The report found fragmented responsibility and insufficient coordination for maintenance and some defences in disrepair.
Mr Johnson added London should learn from the flooding in New Orleans, where the city's flood defences were breached by the effects of Hurricane Katrina.
"In New Orleans, the world has had a tragic reminder of the threat from natural disaster and the impact flooding can have," he said.
"It is therefore vital that lessons are learned and action taken immediately to streamline responsibilities for flood defences and planning control in the Thames Gateway."
The report says 5% of East London's defences are in a poor condition, and the situation could be worse towards the coast.
It also claims that private landowners are responsible for maintaining many of the city's 2,400 defence structures - yet in some cases identifying who owns the land is impossible.
The Environment Agency described the report as a "valuable contribution" but added the flood defences were in "good shape".
David Wardle from the agency, who is programme executive for the Thames Estuary, said: "We are pleased that the London Assembly agrees that the very real risk of flooding to which the capital is exposed cannot be ignored.
"But Londoners can be reassured that their city's flood defences are in good shape, offer a high level of protection and will benefit from an investment of some £300m over the next 15 years."
An Agency spokesman added that all defence structures were inspected every six months, and it could carry out required maintenance.