THE UK could get more than two months' rainfall in just a few hours as thunderstorms move in across the country.

Forecasters are preparing for a "worst-case scenario" of up to 150mm this afternoon and this evening in some places - more than twice the 70mm average for the month of August - with heavy rainfall already reported across Devon and Cornwall in the south-west of England.

The worst affected areas are expected to be between Birmingham and Cumbria, from around 4pm today (Monday, August 10), although almost the whole of the UK is covered by a Met Office weather warning between now and Thursday evening.

In the warning covering North Wales, a Met Office spokesman said: "Some severe thunderstorms are likely to develop across parts of the Midlands, Wales and then northern England from late Monday afternoon and through the evening.

"These may contain large and damaging hail, torrential rain, frequent lightning and possibly very strong and gusty winds.

"Whilst many areas will see some heavy rain, hail and thunder at times where the worst of the storms occur some exceptional rainfall totals are possible. 60 mm of rain is possible in an hour with a very small chance of 150 mm in three or four hours."

It follows an intense period of very hot weather which saw temperatures reach 34C at Herstmonceux in East Sussex on Sunday, the fourth consecutive day the thermometers passed 30C in the south of England - although there is every chance hot weather will remain, with temperatures of up to 36C possible across the south coast at the beginning part of the week.

Bonnie Diamond, a meteorologist with the Met Office, said: "There is a pretty broad warning in place today, but there is another for central UK - the Midlands up to the North West of England to about Cumbria, and into Wales - which we are watching really closely.

"We are looking at the potential for some thunderstorms there from about 4pm to the early hours.

"It's a worst-case scenario - a caution, really, for what could happen - but we are looking at a potential for 150mm rain in three or four hours.

"Normally for August you're looking at around 70mm for the month, so it's potentially a lot of rain.

"To get that much it will take some areas seeing frequent thunderstorms, and not just passing through."

The Met Office warned that flash flooding could cause travel disruption and power cuts, but also cautioned about the risks caused by fast flowing or deep floodwater.

HM Coastguard dealt with 340 incidents across the whole of the UK on Saturday - the highest number of call-outs in a single day for well over four years. There were a further 335 incidents on Sunday.