About 13,000 homes are without power after Storm Imogen brought heavy rain and winds of up to 96mph to parts of southern Britain.
Waves of up to 19.1m (63ft) have been measured off the Cornish coast, and winds of 70-80mph felt in many areas.
Rail services are disrupted, some cross-Channel ferries are cancelled and drivers are urged to take extra care.
The Met Office has issued an amber “be prepared” wind warning for Wales, south-west England, London and south-east England and says coastal areas could see giant waves and localised flooding.
BBC Weather forecaster Peter Sloss said there had not been winds so strong across such a wide area of southern Britain since the winter of 2013-14.
Winds of more than 70mph have been recorded in many areas, including 81mph in the Isles of Scilly, 84mph in Pembrey Sands, Carmarthenshire.
The Met Office has confirmed that “phenomenal” sea conditions have been recorded at several points offshore. Defined as waves of more than 14m (46ft), it is the highest level on the World Meteorological Scale.
But Nicola Maxey, from the Met Office, said 4-6m was more likely along shorelines themselves, for example in the Bristol Channel, and those peaks were most likely to occur at low tide and therefore have a smaller impact.
The Environment Agency has nearly 60 flood warnings in place – meaning flooding is expected – and more than 170 flood alerts – meaning flooding is possible. Natural Resources Wales has 17 flood warnings and 23 alerts.
Highways England has issued a severe weather alert, warning drivers – especially those in high-sided vehicles, on motorcycles or considering a trip with a caravan – to take extra care.
Motorists are being urged to be particularly aware when travelling on the M4 and M5 motorways on Monday afternoon.
Forecasters said winds would gradually ease through the afternoon and evening.