Londoners could face a bill of more than �4bn over the next few years in the fight to stop the Thames flooding.

The Environment Agency, which has come up with the figure, blames rising sea-levels for the flood threat.

The Thames Gateway region, east of London, has been identified as the site for up to 100,000 new properties to overcome the region's housing shortage.

But now London Mayor Ken Livingstone is taking advice on how best to protect people and property.

Ultimately a badly built property which is vulnerable to flooding could find itself uninsurable

Malcolm Tarling, Association of British Insurers

Sarah Lavery from the Environment Agency told BBC London: "If we do nothing, I don't think it would be an exaggeration to say that London is unsustainable.

"We would have to start thinking about moving our capital city elsewhere."

Insurers say there is �10bn worth of land in the London area at risk from flooding.

Malcolm Tarling, of the Association of British Insurers, said: "Ultimately a badly built property which is vulnerable to flooding could find itself uninsurable."

Thames Barrier

The Thames Barrier is London's primary defence against flooding.

It will need to be raised by four feet to cope with rising water levels and similar reinforcements will have to be carried out to other down-stream defences.

This will provide protection until the end of the century, but it will cost about �4bn.

In order to pay for the work there may have to be a special flood tax which would probably be added to the council tax bills of those most at risk.

Recent storms caused roads to flood

Also the city is not coping with heavy rainfall.

Thames Water is still recovering from the storms of two weeks ago when some roads were badly flooded.

The only solution the company can see is to dig up the roads and replace the Victorian sewers with bigger pipes.

Mike Tempest, of Thames Water, said: "It could run into billions of pounds.

"Ultimately I guess, as with all these types of issues, the customer would have to pay through their bill."

But action will need to take place soon as Europe has decreed there must be a coherent new flood strategy to protect Londoners by the end of the decade.