Terrorists could attack the Thames Barrier and cause billions of pounds worth of flood damage, according to emergency planners.

The barrier, in Woolwich, is the largest and most important single element of London's flood-defence system.

But a terrorist attack on the barrier, which has been in operation since 1984, could leave a huge swathe of the city exposed to flooding and cause around £30 billion of damage.

And contingency plans are reportedly being drawn up to move key public bodies, including the House of Commons and 10 Downing Street, outside the flood zone.

Greenwich and Woolwich MP Nick Raynsford, who heads the London Resilience Committee, which carries out disaster planning, says the Government is doing an "enormous amount of work" to ensure the capital is "as prepared as it can be" to cope with any eventuality.

He said: "Public safety is our priority and to ensure this we believe sensible contingency planning is vital.

"This is why the Government is making substantial investment to ensure the emergency services, local authorities and Government departments and agencies are prepared to respond effectively to any major incident."

Fourteen people died the last time central London flooded, in 1928. And severe flooding in 1953 caused 300 deaths around the Thames estuary and the east coast.

The Environment Agency, which operates the Thames Barrier, says a severe flood could paralyse the central part of the Tube network, damage fresh water and sewer systems and disrupt power, gas and telephone services.

But a spokesman says the barrier has just completed a major security upgrade to ensure compliance with Government security classifications.

She said: "The Thames Barrier is one of many locations in London which may be considered a potential terrorist target. However, we do as much as we can to protect against that threat.

"The Thames Barrier is only one part of the Thames tidal defence infrastructure. It provides the highest level of flood defence in the whole of Europe."

"If there was a terror attack, flooding would also depend on what the weather conditions are," she added.