BRITAIN might have to block off the whole River Thames with a barrier stretching from Essex to Kent if London is to be protected from rising sea levels, according to a new study.

The dyke would stretch up to 10 miles across the Thames estuary from Sheerness, in north Kent, to Southend in Essex, making it one of the biggest engineering projects Britain has undertaken.

The proposal comes in a study by four respected academics on how to protect London against rises in sea level caused by climate change. Details will be revealed at a scientific conference on the climate to be convened by Tony Blair next month to mark Britain’s forthcoming EU presidency.

The researchers found that without a sharp reduction in greenhouse gas emissions, sea levels could rise far faster than the 2ft or 3ft already expected by 2100. They warned that London, without extra sea defences, faces inundation.

Jim Hall, professor of earth systems engineering at Newcastle University, one of the authors, said: “We wanted to look at the more extreme but still plausible scenarios for sea level rise. The chances of these happening are small but the consequences are so dramatic that we have to prepare for them.”

A key finding was that if the city’s existing defences were to be breached, then Westminster and other parts of central London could be 6ft deep in water within an hour.

The report said: “Such rises pose a serious threat to human life particularly when considering the high population density of the Thames estuary. The risk of inundation of many central London hospitals adds further to this strain.”

Under the scheme, a giant barrier would be built between Shoeburyness, east of Southend, and Sheerness on the Isle of Sheppey.

The barrier would be the largest in Britain but smaller than some of those constructed by the Dutch, who use such techniques to reclaim land from the sea. It would contain numerous gates to allow water to flow in and out of the Thames estuary according to the tides, but engineers would also be able to shut the gates if a flood seemed likely. The barrier might also include a road and hydroelectric power generators.

Tim Reeder, project scientist for the Thames Estuary 2100 project, which was set up by the Environment Agency to assess flood risk and river management over the next century, said: “This (barrier) would become an option especially if sea levels started rising faster than now. This is something we cannot predict yet, though we hope it is unlikely. We will report in 2008.”

Historically, London has always been at risk of inundation. Samuel Pepys, the 17th-century diarist, recorded one flood that left Westminster under 6ft of water. London has sunk by more than 3ft since Pepys’s day, so such floods would now be a regular event but for the Thames barrage and other defences including 17 smaller barriers, 240 floodgates and 200 miles of sea walls installed since the 17th century. Despite such defences, central London was still easily flooded by a tidal surge in 1928, when 14 people drowned.

The capital also had a narrow escape in 1953 when a tidal surge caused 300 deaths around the Thames estuary and the east coast. Realisation of the devastation it could have caused if it had reached London prompted construction of the Thames barrier but it took until 1983 for this to become operational.

By 2030 it will have become inadequate because London is still sinking at a rate of about 8in a century while the global sea level is rising. The current prediction is for sea levels to rise by 2ft to 3ft by 2100 but it could be more if the Antarctic ice cap starts melting. There are indications this may be happening.

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