Lashing rain and fierce winds have battered parts of south west Britain as the remnants of Hurricane Bertha sweep across the country.
Winds gusting at almost 50mph struck Cornwall this morning, while nearly a month's rainfall was dumped over parts of Wales in just one night.
The former hurricane tore through the Caribbean early last week and has now brought Britain's warm, balmy summer to an abrupt end.
Torrential downpours broke out over Wales, where 31mm of rain fell on the port of Milford Haven in the 12 hours until 7am - the average rainfall for the whole of August is 40mm.
There are fears this could trigger further floods, and 13 alerts are in place across the south west, south east, east of England and the Midlands.
Met Office yellow weather warnings telling people to "be aware" of rainfall for England, Wales and Scotland have now also been extended to Northern Ireland.
Rachael Vince, a forecaster with MeteoGroup, said: "It is more of an autumnal storm. The rainfall is fairly persistent over the Midlands, north east and north west of England. While there is more showery rain in the south and west of England and Wales."
She said most of Britain could be hit by 25mm of rain over the day, while the mountainous beauty spot of Snowdonia in Wales could be deluged by 50mm of water.
Britons were warned to batten down the hatches as powerful gusts reaching up to 60mph are expected to hit the south west, south and east of England as the eye of the storm moves across central England and up to the north east.
The Red Cross has mobilised hundreds of volunteers who are on standby to help anyone affected by the storm.
Turbulent weather already wreaked havoc across the east coast of Britain on Friday night, leaving many homes flooded and causing power cuts in 1,400 properties.
Fire crews in Norfolk, Cambridgeshire and Lincolnshire battled to pump gallons of water out of swamped properties, while marooned drivers had to abandon their cars as they became stuck in flooded roads.
Ms Vince said that after basking in a glorious heatwave for much of July, the curse of the typical British summer had struck again.
She said: "We did have that heatwave in July, so it is feeling distinctly more autumnal this week as a whole.
"But it is a typical British summer - really a bit of rain in August is nothing unusual.
"The centre of the storm will have passed over the coast of Britain by 5pm or 6pm. But we are in an unsettled spell of weather for this week with showers and perhaps some thunderstorms through the rest of the week."
Temperatures will hit highs of 23 degrees centigrade in the south east today, and hover in the mid-teens in the north of England and Scotland.
But the wind and rain will make most of the country feel far cooler.
The start of a prestigious yacht race has been postponed until tomorrow because of the weather, organisers said.
The Sevenstar Round Britain and Ireland Race was due to start in Cowes on the Isle of Wight today, but will now begin at 9am tomorrow.
The Royal Racing Club, which runs the event, had already changed the route from clockwise to anti-clockwise around the British Isles yesterday because of Bertha.
In a statement regarding the postponement this morning, it said: "The race committee took this decision after receiving advice that the low pressure system known as Bertha is moving more slowly than previously predicted, with the result that the forecast winds for the start and the immediate period afterwards includes sustained winds of 40 knots (46mph) with gusts in excess of 50 knots (57mph) in the English Channel.
"The advice is that this delay will allow time for the severe winds to abate as the low pressure system moves north east."
A cycle race in London and the south east has also been affected by the wind and rain.
The RideLondon Surrey Classic, which sees more than 20,000 cyclists go through rural Surrey before ending on the Mall, has been shortened from 100 miles to 86.
Organisers said they "reluctantly" made the decision after information from the Met Office. The route will now miss out a section covering Leith Hill and Box Hill in Surrey due to predictions of heavy rain.
Natural Resource Wales has issued four flood alerts for the country.
It advised people to stay away from the Usk Estuary and Wye Estuary in Monmouthshire, and the River Llynfi and Ogmore in Bridgend.
It also advised people should take care near rivers in the Ewenny and Vale of Glamorgan areas as rain was expected to fall heavily until lunchtime.
Fire crews in Mid and West Wales have been called to three flooding incidents in St Davids, Pembrokeshire.
The Environment Agency has issued three flood warnings for the North Sea coast.
High tides coupled with predicted strong winds led to concerns about "overtopping" for the area around the Spa Complex in Scarborough, North Yorkshire, Harbour Road in Bridlington and parts of the North Bank of the Humber Estuary.
The risk was expected to be worst around teatime.
The last day of the Boardmasters music and surfing festival in Cornwall has been cancelled, organisers said.
The event at Watergate Bay, near Newquay, was halted "in light of the extreme weather conditions caused by hurricane Bertha", they said in a statement.
They added: "Our guests, staff and performers' safety is our main concern and we have decided that the magnitude of the conditions is too severe for the arena site to be opened."
The international surfing competition will go ahead at Newquay's Fistral Beach as planned, they said.
Brittany Ferries said it had cancelled its four fast-craft catamaran services between Portsmouth and Le Havre and Cherbourg in France today.
However, it said all passengers would be able to travel on its conventional ferry services which are operating normally.