England and Wales will be hit by a "significant storm" on Sunday night with winds of up to 80mph in some areas, the Met Office has warned.
Forecasters say the storm is expected to bring "exceptionally strong winds" to parts of the UK when it strikes late on Sunday night.
There were hopes the huge storm brewing over the Atlantic Ocean would miss the UK, and sweep instead through the English Channel.
But meteorologists are now certain it will strike the south west of England and could leave a trail of destruction as it gusts north-east across the country, causing structural damage, and bringing down trees and power lines.
The Met Office said winds of between 60mph and 80mph are expected across the southern half of the UK.
In exposed coastal areas the winds may reach speeds in excess of 80mph.
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."
Roads may also be hit by flash flooding, bringing rush hour traffic on Monday morning to a halt, and homes could be flooded.
The Environment Agency says 20-40mm of rain could fall within six to nine hours.
Severe weather alerts are in place for England and Wales, with an amber warning, meaning "be prepared", for the southern half of England and Wales.
There is a lesser yellow warning, meaning "be aware", for the rest of Wales and England up to the border with Scotland.
Sky News weather presenter Jo Wheeler said: "The storm is coming through at the worst possible time; from around midnight to midday on Monday - which takes in the rush hour when people are trying to get to work.
"Winds gusting at 80mph are quite capable of bringing down trees and power lines and causing structural damage.
"Trees are still full in leaf at this time of year and the ground is quite damp so there is more chance they can be ripped from the ground, and branches ripped from trees.
"There's also the possibility of flooding with torrential downpours and leaves blocking drains. South Wales and southern England are most at risk.
"There is likely to be flash flooding on roads and the AA will be out in force."
She said Atlantic storms of this type usually develop further west across the ocean, losing strength by the time they reach the UK and Ireland.
But the storm will develop on Saturday and intensify on Sunday fairly close to the coast before it strikes with full force.
A strong jet stream and warm air close to the UK are contributing to its development and strength.
Winds could top 80mph as they hit seaside towns and villages before slowly losing power as they gust across the UK and out to the North Sea.
Some have compared its potential to the Great Storm of 1987 and record-breaking gales in south Wales in 1989.
However, it will strike two weeks later than the 1987 storm, which left a trail of destruction on October 15 and 16.
Veteran weatherman Michael Fish famously failed to predict its severity before it flattened trees, knocked out power and left 22 people dead in England and France.
This time he warned people to "batten down the hatches" and keep checking the forecasts as the powerful storm approaches.
Darron Burness, head of the AA's flood rescue team, said: "The timing couldn't really be worse, potentially causing significant travel disruption on Monday morning, which is one of the busiest times on the roads.
"If it's bad where you are, keep tuned to the weather and traffic reports - in case of road or bridge closures - and heed any local police advice about whether it's safe to travel.
"At a minimum, take a fully-charged mobile phone and warm, weatherproof clothing."
He said the AA's specialist flood rescue team, driving modified Land Rovers, have been working all week and are on stand-by.
The Metropolitan Police has urged people to avoid calling 999 during the storm unless there is a real emergency.
A spokesman said: "The Metropolitan Police Service is reminding Londoners to dial 101 when contacting police when it's not an emergency with expected increases in demand brought about by high winds in the next few days."