The Met Office has upgraded its weather warning for Storm Francis to amber with “very strong winds” expected to bring disruption and threaten lives.
Forecasters had previously put out a yellow warning for the winds, but increased the threat level on Tuesday as extreme weather became more likely.
The amber warning, which covers most of Wales and a swathe of central England, runs from 2pm until 10pm on Tuesday.
Forecasters said there will longer journey times, public transport services could be cancelled and road, rail, air and ferry links could be closed amid 60 to 70mph winds and heavy rain.
Storm Francis - In pictures
Gusts of 64mph were recorded at the Needles, off the coast of the Isle of Wight, on Tuesday morning, while 30 to 40mm of rain fell in Northern Ireland.
And there is a “good chance” of power cuts while mobile phone coverage could also be affected by the storm, the Met Office said.
People are urged to take care in the high winds amid flying debris that could lead to “injuries or danger to life”. There could be damage to buildings and trees, such as tiles blown from roofs and branches falling on roads.
Lives could also be threatened by large waves and beach material being thrown onto coastal roads, sea fronts and properties, forecasters warned.
Chief Meteorologist Steve Ramsdale said: “Storm Francis arrived early on Tuesday morning, bringing another spell of wet and windy weather for the UK over the next few days.
"Wind speeds this strong are unusual during August and may come as a surprise to people spending time outdoors trying to catch the last few days of summer.
“A number of severe weather warnings have been issued and these warnings can be updated regularly so please keep up to date with the latest Met Office forecast.”