England and Wales face a battering tonight from the worst storm in five years, forecasters warn.
Winds of more than 80mph could leave a trail of destruction across a large swathe of the UK, 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 20-40mm of rain could fall within six to nine hours overnight.
Insurance companies have advised households to take steps to protect themselves and their property.
People should also 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 develop 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.
It 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 last night: "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 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.
A spokesman said: "We are supporting local authorities who will respond to any reports of surface water flooding.
"Seafronts, quaysides and jetties should be avoided due to the risk of overtopping by waves and wind-blown shingle."
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."
Home insurers were bracing themselves for the prospect of a high number of storm damage claims.
The forecast of storms has also caused the cancellation of ferry services today and tomorrow between Plymouth and Roscoff as well as Penzance and the Isles of Scilly.
Brittany Ferries, which runs the Plymouth-Roscoff route, said on its website: "We are in the process of contacting all passengers booked on these sailings. We apologise for the inconvenience the cancellation of these services will cause."
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.
Councillor Mike Jones, chairman of the LGA's environment board, said: "Councils are preparing for the worst while hoping for the best. Local authorities up and down the country are preparing to divert staff from their normal duties and have placed additional employees on standby to work with fire crews and other emergency services to get people help if they need it."
Racecourses in Leicester, Redcar and Bangor have all announced precautionary inspections on Monday morning in light of the weather forecast.
Leicester clerk of the course Jimmy Stevenson said: "It's very much precautionary at this stage."
David Cameron said he has spoken to the organisations responsible for public safety during the storm.
The Prime Minister wrote on Twitter: " I've just chaired a call with various Government departments and agencies to hear about all the plans to ensure people are protected from tonight's storm."
The British Red Cross has teams ready to assist people stricken by the storm, and urged households to prepare for possible blackouts.
Simon Lewis, head of emergency response at the humanitarian organisation, said: " It's important people are ready should the worst happen. Make sure you have torches at hand, as well as a battery powered or wind up radio to keep up to date with weather warnings and information for your area.
"Taking a few minutes to get together an emergency kit can make a huge difference."
Network Rail warned there is likely to be disruption to trains from fallen trees and localised flooding on Monday if the weather forecasts prove to be accurate.
Robin Gisby, Network Rail's managing director of network operations, said: "We are monitoring the forecast closely and have put in place agreed industry plans for dealing with difficult conditions such as these.
"Our maintenance teams will be out over the weekend to check that key drainage points are clear and we will have additional teams available across the network to clear fallen trees and repair any damaged equipment as quickly as possible to allow train operators to keep services running safely. As ever, safety is our top priority."
A number of operators will be running revised timetables on Monday in response to the conditions.