Local resident Daniel Bidgood was in his house when the tornado, which he said was about 20 metres (yards) across, smashed his windows.

"It was very large and certainly very powerful," he told BBC television. "You could see it ripping up heavy chunks of mortar and smashing it into cars."

One man in his 50's was taken to hospital with head injuries. Five other adults were treated for shock and minor injuries after the tornado struck at around 1100 GMT.

Around 100 houses were damaged, a Fire Brigade spokesman said, "so I would imagine several hundred people may be displaced". Some of those evacuated were being housed in local churches.

More than 20 fire engines were sent to the scene and the area was cordoned off.

"SEVERE"

The London tornado was rated at T4 on a scale of 0 to 10. This means winds were moving at between 115 and 136 mph (185-219 kph), rating the tornado as severe, said a spokesman for the Meteorological Office.

Britain experiences between 30 and 40 tornadoes in an average year, he said, but they rarely hit built-up areas. In July 2005, a tornado in Birmingham, central England, damaged dozens of homes.

The cost in London could run into millions of pounds. Houses in the area cost an average 550,000 pounds (US$1.08 million), a local estate agent said. The Association of British Insurers said most home insurance policies would cover the costs.

"You're not likely to find the word 'tornado' in a UK insurance policy," a spokesman said. "But it would be covered under 'storm damage'."

Tornadoes are a vortex of swirling air caused by a build-up of heavy thunder clouds. The Met Office spokesman said it was impossible to tell if global warming had a role to play in the London tornado.