Three million people have been urged to stay indoors and avoid driving as Storm Darragh could bring winds of up to 90mph.
The Cabinet Office issued an emergency alert at 6.45pm to people in areas covered by the Met Office’s rare red warning for wind in parts of Wales and south-west England.
The Emergency Alert system sent a message to every compatible mobile phone in the impacted areas, containing information about the red warning and guidance on how to stay safe into Saturday.
“Stay indoors if you can. It is not safe to drive in these conditions.”
It was the largest use of the emergency alert system outside a test scenario – the last test being in April 2023.
It is understood residents in: Devon, Bath and North East Somerset, the City of Bristol, North Somerset, South Gloucestershire, Somerset, Isle of Anglesey, Gwynedd, Conwy, Ceredigion, Pembrokeshire, Carmarthenshire, Swansea, Neath Port Talbot, Bridgend, Vale of Glamorgan, Cardiff, Monmouthshire and Newport received the alert.
Mobile phones made a loud siren-like sound even if they were set on silent. The sound and vibration lasted for about 10 seconds.
Red is the most serious weather warning the Met Office can issue, meaning dangerous weather is expected and people are urged to take action to keep themselves and others safe.
The warning, which has led to the cancellation of events including Christmas attractions, is in place from 3am to 11am on Saturday.
Forecasters say the strongest winds will begin to ease from late morning.
A separate amber warning covering a larger stretch of the west coast of the UK, stretching from southern Scotland to Cornwall, and Northern Ireland is in place from 1am until 9pm.
Flying debris and falling trees could pose a risk to life, while large waves and beach material could be thrown on to coastal roads and seafronts.
There could also be damage to buildings and homes, with roofs blown off and power lines brought down, as well as power cuts affecting other services such as mobile phone coverage.
The fourth named storm of the season is also expected to bring heavy rain over the weekend, with more than 100 flood warnings and alerts in place across the UK.
An amber warning for rain is in place in Wales from 3am to 6pm on Saturday with heavy rain likely to lead to disruption to transport and infrastructure.
A yellow warning for rain is in place for Northern Ireland and Wales, both of which were badly affected by flooding during Storm Bert, as well as parts of Scotland from 3pm on Friday until noon on Saturday.
Periods of heavy rain in South and mid-Wales through Saturday are likely to see 20-30mm fall in three to six hours, with totals of 80-90mm possible by the time it begins to ease in the evening.
Welsh Deputy First Minister Huw Irranca-Davies warned there may be very significant impacts from the storm and urged people to take extreme care this weekend.
Stena Line said the storm would be “impacting” some sailings over the coming days, and Bristol Airport said disruption was expected.
National Rail said the storm was likely to affect services across the whole network, advising people to check their journey in full before travelling as it may mean last services of the day are cancelled and passengers may not reach their destination.
Chiltern Railways advised customers to only travel if absolutely necessary on Saturday with significant disruption expected across the network.
A reduced timetable will be in operation with one train per hour on all routes. Chiltern said customers with tickets on Saturday can use them on Sunday, Monday or Tuesday instead.
Up to 60mm of rain could fall in these areas, which may lead to some flooding and disruption, forecasters said.
Rhondda Cynon Taf, where between 200 and 300 properties were flooded during Storm Bert last month, is set to be hit by heavy rain again.
Longleat Safari Park in Wiltshire said on its website that it would not open on Saturday for safety reasons.
In London, all the Royal Parks, Brompton Cemetery and Victoria Tower Gardens will be closed on Saturday.
A spokeswoman said the closures included Hyde Park Winter Wonderland as well as all park roads and cycleways, cafes and kiosks, Park Sports venues, the Serpentine Lido and Boating Lake, and the Royal Parks Shop.
The Football Association of Wales said all matches in the country were being postponed on Saturday.
In racing, Sandown’s Betfair Tingle Creek Chase meeting and the Boylesports Becher Chase fixture at Aintree on Saturday must pass precautionary inspections due to the threat of high winds.
Belfast City Council said the city’s Christmas market would close on Saturday due to the weather warnings, while the Enchanted Winter Garden events at Antrim Castle Gardens had been cancelled for Friday and Saturday.
The Environment Agency said that while the more severe impacts from the storm were likely to be wind-related, minor flooding was likely in parts of the North East and the West Midlands over the weekend.
Red warnings are issued “relatively infrequently”, according to the Met Office.
Since the Met Office began issuing red warnings in 2011, there have been 20, with typically one or two a year.