England and Wales will be battered by the worst storm in years with heavy rain and hurricane-force winds expected tonight and tomorrow morning, forecasters warn.
Winds of more than 80mph could leave a trail of destruction, bringing down trees and causing widespread structural damage, leading to power cuts and transport chaos tomorrow morning.
Surface water floods could strike much of England as the Met Office predicts 2-4cm of rain could fall within six to nine hours overnight.
David Cameron said he has spoken to the organisations responsible for public safety during the storm.
Insurance companies have advised households to take steps to protect themselves and their property.
Direct Line said p eople should establish evacuation plans, place valuable items upstairs to limit flood damage and ensure gutters are clear so water can drain away.
The storm has been named St Jude after the patron saint of lost causes, whose feast day is tomorrow.
It will travel over the Atlantic and is expected to hit the South West late tonight, before moving north-eastwards across England and southern Wales.
Heavy rain will accompany it, with strong winds in the early hours of tomorrow, but the storm is expected to have moved out over the North Sea by lunchtime, leaving strong breezes in its wake.
The Met Office described the storm as not one "you would see every year", and said the expected wind strengths would be similar to storms in March 2008, January 2007 and October 2000.
Gusts of 115 mph were recorded during the Great Storm of 1987, when 18 people died and t housands of homes were without power for several days.
The Met Office has issued an amber warning, meaning ''be prepared'', for the southern half of England and the southern half of Wales.
It gave a lesser yellow warning, meaning ''be aware'', for the rest of Wales and England up to the border with Scotland.
Frank Saunders, chief forecaster at the Met Office, said: "We are confident that a severe storm will affect Britain on Sunday night and Monday. We are now looking at refining the details about which areas will see the strongest winds and the heaviest rain.
"This is a developing situation and we'd advise people to stay up to date with our forecasts and warnings over the weekend, and be prepared to change their plans if necessary. We'll continue to work closely with authorities and emergency services to ensure they are aware of the expected conditions."
Atlantic storms of this type usually develop further west across the ocean, losing strength by the time they reach the UK and Ireland.
But this is expected to appear much closer to land, potentially moving across the country while in its most powerful phase.
A strong jet stream and warm air close to the UK are contributing to its development and strength.
Chris Burton, a forecaster with MeteoGroup, the weather division of the Press Association, said: "We are going to see some heavy and persistent rain heading north across England and Wales overnight from early this evening, from the South West.
"As the rain pushes north the winds will pick up, and by midnight there should be gusts of about 60mph across south-west England.
"Through the early hours much of southern England will see winds of 60-80mph, maybe closer to 90mph in exposed areas on the south coast.
"Further north, in south Wales, the Midlands and East Anglia these will be between 50mph and 70mph.
"The storm is going to move through very quickly, and as we go into Monday morning the winds will ease off from the South West, and by lunchtime most areas will start to see the winds ease off and die down.
"Rains will also clear through to the east through the morning, leaving a few showers in the afternoon."
The Prime Minister chaired a conference call with various Government departments and agencies this morning on plans to protect people from the storm.
Transport ministers, the Department for Communities and Local Government and the Cabinet Office updated Mr Cameron on preparations and contingency plans for transport, local authorities, schools, health and power supplies.
He was told that the storm could have a widespread impact but plans were in place to respond.
The Environment Agency has teams working to minimise river flood risk, clearing debris from streams and unblocking culverts, and are closely monitoring water levels so they are ready to issue flood warnings if necessary.
Martin Hobbs, head of asset resilience at the Highways Agency, said: "Be aware of sudden gusts of wind and give high-sided vehicles, caravans, motorbikes and bicycles plenty of space."
The Local Government Association (LGA) said local authorities would divert staff from their normal duties to help out with emergency relief efforts if required.
They have found emergency accommodation should families be evacuated from their homes, and highways teams are on standby to rescue stranded motorists and clean debris from roads.
Network Rail warned there is likely to be disruption to trains from fallen trees and localised flooding tomorrow if the forecasts prove to be accurate.
Passengers are advised to visit www.nationalrail.co.uk for the latest information.
Home insurers were bracing themselves for the prospect of a high number of storm damage claims.
Rob Townend, claims director at Aviva, said: "We have drafted in extra staff into our contact centres so we are poised and ready to help all those who might need us if the worst happens."
Portsmouth seafront was closed from 3pm today until further notice as the storm approaches.
A spokesman for the city council said: "Pedestrians and vehicles will be prevented from direct access on the seafront for their own safety, due to the high winds."
Hampshire Police advised residents in the county and on the Isle of Wight to take precautions as winds are expected to hit 80mph.