The large-scale development planned for the Thames Gateway is at risk of flooding unless immediate action is taken to protect it.
The GLA has told mayor Ken Livingstone that he must significantly modify his draft London Plan to meet the threat of rising water levels.
A committee investigating the problem is set to publish its findings in the next few weeks. It will call for planned housing developments to be cut back and replaced with green open space to serve as a flood plain.
The mayor is pushing development in the Thames Gateway as key to the solution of London's housing problems. The draft London Plan proposes the creation of 80,000 new homes during the next 20 years.
However, Roger Evans, chair of the GLA's flooding scrutiny committee and Conservative member for Havering and Redbridge, said the area was in severe danger from tidal flooding. The committee heard evidence from the Environment Agency, which expressed grave concerns that the Thames' flood defences were insufficient to protect the area, he said.
'There needs to be a decision, ' Evans said, 'either to construct very high flood defences or to find somewhere for the water to go.'
In addition to the creation of flood plains, it may also be necessary to construct a second Thames barrier, he added.
Other recommendations in the report, which examines the risk of flooding city-wide, includes the tightening up of regulations to demand the use of sustainable drainage systems on all new developments.