The worst storm for more than five years is expected to batter England and Wales on Sunday night and Monday.
The Met Office said winds in excess of 80mph could bring down trees and cause structural damage, leading to power cuts and transport disruption in rush hour on Monday morning.
The Environment Agency warned of surface water flooding across most of England as forecasters from MeteoGroup predicted up to 25mm of rain could fall in just six hours.
The storm will develop over the Atlantic today and is expected to hit the south-west of Britain before moving north-eastwards. Heavy rain is due to arrive on Sunday night with strong winds in the early hours of Monday.
It has been named the St Jude Storm after t he patron saint of lost causes whose feast date is on Monday.
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 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.
Steve Willington, chief forecaster at the Met Office, said: "We are talking about a storm which doesn't yet exist, so there remains some uncertainty about its possible timing, track and strength. However, several forecast models currently suggest we will see a significant storm with exceptionally strong winds impacting parts of England and Wales.
"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.
Gemma Plumb , forecaster with Me teoGroup, the weather division of the Press Association, said: "The first place that will see the strong winds will be south-west England, which could see gusts of 75-80mph.
"The storm will move north-eastwards, with winds of 60-70mph in the Midlands and East Anglia.
"Northern England and North Wales are looking at winds of 50-60mph."
Darron Burness, head of the AA's flood rescue team, said: ''If the predicted storm strikes, 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.
''Strong wind and torrential rain is an unpredictable and hazardous combination, which can be quite overwhelming when you're driving.
''There's likely to be tree and other debris on the roads as well potential flooding, so it's very important to keep your speed down and drive with great care, particularly on country roads early on Monday morning when it's still dark."