Swathes of Britain could be left totally underwater by a tsunami in the Atlantic Ocean, it has been claimed.
An earthquake off Portugal could trigger disastrous levels of flooding along the coast of the UK, scientists have said.
The Isles of Scilly and parts of Cornwall would be worst hit if there were a repeat of the Lisbon earthquake of 1755.
Experts believe that a quake similar to the devastating one 250 years ago would send a 10ft wall of water towards the UK.
The tsunami would crash over the tip of Cornwall within a few hours leaving much of the coast underwater and wiping the tiny Scilly Isles off the map.
The prospect of the natural disaster was raised at a meeting of the Devon and Cornwall Local Resilience Forum, which includes police, town planners and emergency services.
Members of the LRF told the meeting they fear Britain could suffer its own version of the 2004 Boxing Day Tsunami. They want an early warning system to alert bathers and people living on the coast, similar to technology used across Asia and America.
Paul Netherton, assistant chief constable of Devon and Cornwall Police and LRF chairman, said other countries have suffered by failing to plan ahead.
He told the forum: After seeing the devastation in Japan and Thailand we have to ask the question of the scientific community what and where would be the impact, is it likely and, if it is, should we plan for it?
A lot of areas would cope but in others it would cause a big problem. We have to consider whether we do anything differently than we do now for severe weather events.
Tsunami are different you don’t know they are coming and the water keeps coming and coming.’
Britain was last hit by a tsunami in 1755, the year of the great Lisbon earthquake, the meeting in Bristol was told.
In the event of an earthquake in the Atlantic, the public would currently rely on the British Geological Survey to register the quake and the arrival of a subsequent wave at nearby Portugal.
Neil Hamlyn, LRF coordinator in Devon and Cornwall, said: The Isles of Scilly could be affected greatly, the islands could be covered and disappear.’
The 1755 Lisbon earthquake almost totally destroyed Lisbon and adjoining areas and had a magnitude in the range of 8.5 on the Richter scale.
Under the Civil Contingencies Act of 2004, the UK maintains a National Risk Register of Civil Emergencies.
The very high risk table is currently topped by pandemic flu, terrorist attacks and coastal flooding, though the last major civil flooding emergency was on the east coast in 1953.
Mr Hamlyn says a tsunami is in the top 20 of the second category of high risks.
Other uniquely regional threats include a nuclear submarine incident at Devonport dockyard and possible contamination from old tin mines in Cornwall.
Work has also already been done to predict the catastrophic impact should a massive slab of rock twice the volume of the Isle of Man break away from the island of La Palma, in the Canary Islands, and smash into the Atlantic Ocean.
The resulting tsunami, higher than Nelson’s Column and faster than a jet aircraft, would devastate the eastern seaboard of America and inundate much of southern Britain.
Associated Newspapers Ltd