The UK's weather agency issued a rare red alert for high winds as the government warned three million Britons to brace themselves for the impact of Storm Darragh.

Red is the most serious warning that meteorologists can issue and is used "relatively infrequently", according to the Met Office.

The alert covers parts of Wales and the southwest of England and is in place from until 11am on Saturday.

The Met Office warned of "damaging winds", with gusts of 145 kilometres per hour possible over coastal and hilly areas.

Darragh, the fourth named storm of the season, is also expected to bring heavy rain, with more than 100 flood warnings and alerts in place across the UK.

Approximately three million people living in the area covered by the red warning were to receive a siren-like alert on their phones warning them of the expected high winds, the government said.

"This will be the largest ever use of the (emergency alert) system outside a test scenario," a spokesperson added in a statement.

They said that the sound and vibration would last for about 10 seconds, even if people's phones were on silent.

Darragh comes two weeks after Storm Bert battered much of Britain, causing "devastating" flooding in parts of Wales and knocking out power to thousands of homes in Ireland.

Climate scientists say that storms are becoming more powerful as the world heats up due to climate change driven by burning fossil fuels.

Warmer ocean surfaces release more water vapour, which provides additional energy for storms, strengthening winds.