This dramatic aerial picture shows the devastation along the Thames of one of the worst floods for a century.

Low lying districts east of Tower Bridge appear to be devastated along the riverfront and several miles inland.

All the low-lying areas of East London are under water—the whole Isle of Dogs, Wapping, Limehouse, Poplar and Blackwall are all submerged, along with Canning Town, West Ham, Silvertown and Becton.

The south side is no better, with Bermondsey, Rotherhithe, Deptford, Greenwich and Charlton under water.

It couldn’t be worse—but that’s what the Environment Agency has come up with for a national flood alert aimed at testing the response of the emergency services in a ‘worse case’ scenario.

Operation Watermark from today until Thursday involves Government ministers in mock emergency meetings with the utilities such as Thames Water and the gas and electric companies to measure how they would also cope.

“This is Britain’s biggest-ever emergency exercise for us to test our response,” said Environment Minister Richard Benyon. “More extreme weather and rising sea levels mean we have to be prepared to deal with the impact of major flooding.”

Flood defence schemes along the Thames are going ahead with cash from the Environment Agency.

Its flood defence committee, made up of local authorities including Tower Hamlets, is putting up �10 million to reduce flood risk in low-lying areas.

Its chairman Amanda Nobbs said: “Every �1 spent on protecting communities saves �8 on repairing damage afterwards.”

One-and-a-half million people are at risk of flooding from the Thames and its tributaries like the River Lea, as far inland as Old Ford and Hackney Wick.

A ‘flood awareness’ day is also being held on Thursday when the Thames Barrier is open to the public, with a three-day exhibition opening the following day.