Two motorists were killed as storms with winds up to 124mph hit Scotland, Northern Ireland and northern England.
Flooding in Ronaldsay, Orkney, is thought to be the worst in 20 years
Around 76,000 people were left without electricity in Scotland after winds downed trees and telegraph poles.
Two lorries were blown over in Scotland and Northern Ireland, killing one driver and another motorist.
A rescue operation was under way off the coast of Scotland after a fishing boat with 19 Spanish and Portuguese crew aboard lost power.
The crew members on the Spanish vessel were unharmed but coastguards warn they are still at the mercy of 30ft (10m) high waves.
The BBC Weather Centre said the worst storms have passed but warned of swollen rivers and blizzards. The Met Office said unnecessary travel should be avoided because of fallen trees, damaged buildings and coastal flooding of roads.
The strongest winds were measured at 124mph on North Rona, in the Western Isles.
Winds of at least 70mph have hit most of Scotland, along with rain, sleet, thunder and lightning.
Across Scotland roads are blocked, main bridges are closed, ferry services have been abandoned and rail routes suspended.
By Wednesday lunchtime, 11 flood warnings were in place in England and Wales and 42 flood watches.
In Scotland there were four flood warnings and 19 flood watches.
Climate change 'reality'
Environment Minister Elliot Morley told the House of Commons that "detailed weather forecasts" had enabled public and private bodies to take steps to reduce the damage caused by storms.
He said this included the cancelling of train and ferry services.
Mr Morley told MPs unpredictable weather was something the country now had to prepare for.
"Climate change is a reality," he said. "We have had the warmest 10 years on record, since the 1990s. "We have had double the extreme weather incidence in the last decade.
"And it does mean that we have to take all steps necessary in terms of reducing risks to people, bearing in mind that we cannot guarantee that extreme events won't happen."
The weather has hampered salvage efforts in Carlisle, where thousands of homes were evacuated due to flooding at the weekend.
Three people were killed and two went missing after torrential rain and gales swept north England and elsewhere.
Environment Minister Elliot Morley has announced a review of the flood warning system as many people had not been alerted to the danger early enough.
In the worst affected areas:
- One driver was killed when his car was crushed by a lorry blown over on the A1 near Burnmouth, in the east of Scotland.
- A van driver died when his vehicle and a lorry were involved in a collision on the A90 northbound in Tayside.
- A lorry driver died after his vehicle was blown off Foyle Bridge in Londonderry, Northern Ireland. The bridge has been partially re-opened.
- Around 1,000 homes were without power in Northern Ireland, mostly in rural areas of County Armagh and Down.
- Up to 10,000 homes in Hexham, Northumberland, are without running water.
- There are still 4,000 properties in Cumbria without electricity.
- Buildings at Sellafield nuclear plant suffered external damage and staff have been sent home until Thursday.
Call the Environment Agency's Floodline for flood warnings on 0845 988 1188.
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