The Thames Gateway guru claimed the Tube system could be inundated with flood water unless immediate action is taken, including building a massive new Thames barrier near Tilbury.
Farrell made the surprise outburst during a debate about the Thames Gateway plans at a recent meeting of the London Assembly's planning and spatial development committee.
Speaking alongside Richard Rogers, London Mayor Ken Livingstone's advisor on urbanism and architecture, Farrell said the 'real cost' of failing to plan for flooding in the Gateway area was to 'London itself'.
He said: 'There is a New Orleans-style threat to the Tube system, where the water comes over the embankment wall and starts flooding the Bakerloo Line.
Farrell went on to say: '[There will be] serious, serious costs of overcoming these disasters.'
Last night Farrell repeated the claims at the RIBA's Portland Place headquarters, where he was being named as the new design champion for the Medway area of north Kent. He said that the danger of flooding to the Circle Line could 'make the work of terrorists feel like nothing'.
This is not the first time the impact of the devastating floods in New Orleans has
been linked to the dangers facing London and the Thames Gateway.
In September, another adviser to Livingstone, Ricky Burdett, said the American disaster would 'sharpen the focus' on the potential flood risks ( 'Beware of New Orleans repeated in Thames Gateway').
Despite these warnings, Farrell is worried that there are still no coherent plans to tackle the key flooding issues and said that, until there are concrete proposals, further development in the Gateway would be unwise.
He added: 'I think that until the government makes any proper statement about plans for flooding, it would
be foolish to build in an area we [know is] going to flood.'