Parts of the UK were braced for flooding today as torrential rain soaked much of the country, evoking memories of last summer’s deluges which saw hundreds of thousands of people stranded.

The Environment Agency issued eight flood watch alerts as heavy downpours hit Wales, Northern Ireland, the Midlands, East Anglia and London.

Forecasters said every corner of the country was likely to suffer a soaking, as a band of low pressure moved rapidly towards the north-east. About 10mm of rain fell in one hour in London this morning, while more than 20mm was recorded in Devon and Cornwall overnight.

The heaviest rain was in Wales, with Cambridgeshire, Leicestershire and Suffolk also experiencing severe weather conditions.

The Met Office said the rain was caused by a low pressure system and active fronts moving very quickly northwards – unusual for the month of August. It predicted unsettled weather today, marked by frequent showers and heavy thunderstorms.

Claire Austin, a meteorologist from MeteoGroup, said: “There is a possibility of flooding today. The reason we have got so much rain is a band moving north-east.

“Northern Ireland, the Midlands, East Anglia and London have already been hit. There will be heavy rain and, at times, thunderstorms.”

Flood watch alerts were issued for the River Axe in Somerset, the Rivers Taf, Cynin and Ely in Wales, Rivers Otter, Sid and the Middle Exe and Exmouth areas of Devon, the River Cole catchment in the West Midlands, the River Ray catchment in Oxfordshire. In Wales, the Rivers Ely, Taf and Cynon and the Western Cleddau were also under watch.

A severe weather warning was issued for Northern Ireland, with up to 50 mm of rain expected overnight.

Areas under watch can expect flooding of low lying land and roads. The Environment Agency urged residents in affected areas to monitor local news and weather forecasts, charge their mobile phones and prepare to act on their flood plans if necessary.

Police warned of flooding on roads, including on the main A1 Belfast-Dublin Road at Banbridge, County Down.

This morning, three teenage girls were rescued from the county's Mourne mountains after their camp was washed away by a torrential downpour.

The campers called the Mourne rescue team after finding their route down from the mountains blocked by a swollen river.

Scotland has already suffered diabolical weather this month, with heavy rains causing widespread floods and landslides. In Renfrewshire, a dam is to be demolished after water reached dangerous levels for a second time, prompting fears it could burst.

Paul Simons, The Times's weather specialist, said flooding was now likely elsewhere as saturated ground and high rivers meant rain had nowhere else to go. Weather patterns were quite similar to last summer, he said, though rainfall was not likely to be so extreme. However, he added: "There is no real hope of any sustained dry weather on the horizon."

Forecasters predicted the rain would ease somewhat tomorrow, replaced by a mixture of sunshine and showers. While Friday is expected to be sunny, most Britons are likely to see another wet weekend.

About 5 million people live in flood risk areas in England and Wales. The floods of June and July last year were among the worst on record, with 50,000 homes flooded and 350,000 people stranded without clean water for days. Thousands are still living in temporary accommodation. Seven people also lost their lives.