A teenager was feared dead after being swept into the sea as the worst storm in years closed in on England and Wales.
Flights were cancelled and trains suspended as heavy rain arrived in the South and h urricane-force winds of over 80mph were expected to cause widespread disruption and power cuts.
A 14-year-old boy was swept into the sea at Newhaven, East Sussex at about 4.15pm while swimming with friends around 10 metres off shore.
Rescue services including a helicopter launched a search but a Maritime and Coastguard Agency spokesman described conditions as "atrocious".
The so-called St Jude Storm was expected to hit the South West late tonight before moving north eastwards across England and southern Wales.
About 60 flights were cancelled at London's Heathrow Airport tomorrow "to ensure the safety of passengers and employees", a spokesman said.
Many train companies in the South have already announced they will be running amended timetables, with some not operating at all until late morning.
Gatwick, Stansted and Luton airports urged passengers to keep up to date with the status of their flights, although there were not initially any cancellations.
Martin Young, chief forecaster at the Met Office, described the expected conditions as "a major storm".
He added: " This weather system is typical of what we expect to see in winter but as it's coming in during autumn - when trees are in leaf - and while the ground is fairly saturated, it does pose some risks.
"We could see some uprooted trees or other damage from the winds and there's a chance of some surface water flooding from the rainfall - all of which could lead to some disruption."
The Environment Agency issued three flood warnings - where flooding is expected - in the South West, and 128 flood alerts - where flooding is possible - across the whole of England and Wales.
Chris Burton, forecaster with MeteoGroup, the weather division of the Press Association, said: "We're expecting some quite damaging gusts of 60-70mph across southern parts of England and East Anglia.
"They could be 70-80mph on the south coast.
"We've had heavy rain across the South. While the rain moves away the gusts will pick up and spread eastwards. The highest winds will be towards the early hours of the morning."
The storm is expected to move out over the North Sea by late morning, 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 115mph were recorded during the Great Storm of 1987, when 18 people died and thousands of homes were without power for several days.
The Met Office 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.
The high winds caused an 89ft wind turbine to blow over in a field at Higher Rixdale Farm at Luton, near Teignmouth, Devon.
It also saw firefighters dismantle a smaller turbine from the roof of a house in Ilfracombe which was "in a precarious position".
Meanwhile, in nearby Cornwall, a family-of-four escaped unhurt after a tree hit their house.
Two children and their parents were sleeping when the tree crashed through the roof of their house in Camborne at about 1am on Sunday.
Insurance companies 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 was named St Jude after the patron saint of lost causes, whose feast day is tomorrow.
The Prime Minister chaired a conference call with government departments and agencies 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 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 stand-by to rescue stranded motorists and clear debris from roads.