Scientists have been investigating the effects of a 7m-high wave travelling up the Thames, using computer simulations.
The wave was produced by a "virtual storm" as part of a £6.5m project at the UK Met Office.
Researchers say the work is crucial to our understanding of how storms cause dangerous flooding around the UK and the quest to improve forecasting.
The Environment Agency also used the models to test the effectiveness of London's flood defences.
The work is the first of three simulations by the Flood Risk Management Consortium (FRMC), a group of scientists and professionals from universities, private companies and government agencies.
The "extreme, hypothetical storm" created by the group allowed it to test forecasting systems and mathematical models of floods.
The next two projects will simulate a flood on the River Severn and one in the centre of Glasgow.
Extreme Event
The first model was run over three days and was designed to create a storm surge, a localised hump created by a low pressure system and strong winds pushing up the sea. The largest storm surges occur when they coincide with a strong, high tide.
"This event that we deliberately created would have probably overtopped the barrier"
Most events are associated with tropical systems like Hurricane Katrina that battered New Orleans last year. The resulting storm surge was responsible for breaching levees that protected the city from flooding.
In the computer model, the scientists used data from a real storm that hit Britain on 25 November last year. The storm formed quickly and produced winds of up to 85km/h (53mph).
They then superimposed an artificially high tide and exaggerated the model to create a massive surge that swept down the North Sea and up the Thames.
The real storm hit when there was a particularly weak tide, known as a neap tide, so did not cause problems.
Flood Forecasts
However, by using the larger than life model, scientists were able to create the 7m (23ft) wave, the worst of 24 equally possible outcomes.
The wave was approximately three times the size of one that washed into London in 1953, which killed more than 300 people and prompted the construction of the Thames barrier.
"This event that we deliberately created would have probably overtopped the barrier and would have overtopped most of the defences all the way up the river," said Professor Ian Cluckie, chairman of the FRMC.
He added a note of caution that the simulation was "very, very extreme" and that the likelihood of it ever happening was incredibly small.
However the research will help refine flood forecasting techniques and allow scientists to better understand the uncertainties of the weather system in the future.
This is vital as climate change affects sea level and weather patterns, complicating the picture further.
As part of the event the scientists deliberately breached London's flood defences to understand how this affects urban areas, crucial for issuing accurate warnings to the public and emergency services.
Andy Batchelor, Thames tidal flood risk manager for the Environment Agency, believes that the information gleaned from these simulations will help people make the right choice when faced with a flood.
"The natural feeling is that you get out of your house and run," he said. "But in certain areas, running is the worst thing because you are actually going into an area that is going to flood."
The data will also be fed into a longer term project, known as Thames Estuary 2100, which aims to determine the level of flood protection needed over the next 100 years.
The Environment Agency stressed that the latest simulations show that London is adequately protected by its flood defences.
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