Britons have been warned to prepare a 'grab bag' of essentials in case of flooding as torrential rain and thunderstorms hammer parts of the country today, causing mudslides while deluging rail lines and streets.

Firefighters said people should prepare a flood kit, with those living in basement flats or low-lying properties particularly at risk as the heatwave ends with a bang - and up to two inches of rain falling in three hours today.

The worst of the flash floods so far have been seen in staycation hotspots across South West England such as Cornwall, Devon and Somerset, where hundreds of thousands of families are on their summer holidays. 

The A358 in Somerset was closed at Combe Florey after a mudslide last night which saw workers remove more than 50 tonnes of mud and potatoes - and it was still shut this afternoon due to the amount of debris.

Diversions were put in place, and council workers joked they were 'working hard to ensure no chips on drivers' windscreens'.

Families braced themselves for traffic chaos and power cuts as forecasters warned of dangerous flash floods - while authorities told people to prepare the emergency bag of vital belongings in case their homes are damaged.

The National Flood Forum said a flooding 'grab bag' should contain insurance and other important documents, jeepininmidwest.com mobile phones and chargers, emergency cash and credit cards, and any medication or prescription forms.

The London Fire Brigade said people should be 'prepared and know what to do should a flood occur' and 'use sandbags to limit the water flow and move belongings to a higher level'.

The Met Office said people living in 'low-lying properties' should make sure their valuable items are 'ready to go', or 'on a higher level of your house'.

The guidance will apply to millions of people - with Environment Agency the estimating that 5.2million homes in England, or one in six properties, are at risk of flooding.

This includes 2.4million at risk from rivers and the sea, 1million of these also susceptible to surface water flooding and a further 2.8million at risk of surface water alone. 

Dramatic videos showed fast-flowing floodwater cascading down steep roads and deluging junctions in Devon and Cornwall - while Londoners hid under trees, blankets and bin lids to avoid sudden bouts of torrential rail. 

But while the heatwave has now technically ended after eight days today, it remains very warm in much of the UK with highs of 27C (81F) expected this afternoon, before 25C (77F) tomorrow and Thursday, and 24C (75F) on Friday.

The whole of England and Wales has been put under a thunderstorm warning for today, with a separate rain alert in place for Scotland.

There is also a further thunderstorm warning covering all of southern England tomorrow.

The Met Office said some places will stay dry today, but others will see thunderstorms with torrential rain - and flooding could happen quickly, with damage to buildings from water, lightning strikes, hail or strong winds.

Forecasters said fast 'flowing or deep floodwater is possible, causing a danger to life' and that the conditions could cause delays to trains and along with difficult driving conditions as well as some road closures.

Speed restrictions were imposed on some rail routes in Scotland today due to the heavy rain - with train services affected between Glasgow and Inverness, Perth, Dundee and Aberdeen. Flooding was also seen near Perth. 

The Met Office said thunderstorms and areas of heavy rain will develop quite widely across much of England and Wales throughout today, and the extent of these thunderstorms will be more widespread than yesterday.

Up to 1.2in (30mm) of rain is possible within an hour, but where areas of thundery rain become slow-moving, some places could see 2in (50mm) in less than three hours - and even higher totals in a few spots over the whole day.

The stormy weather comes after weeks of soaring temperatures - with at least 30C (86F) recorded for eight days in a row up to and including yesterday, and an unprecedented 40.3C (104.5F) last month.

The official heatwave threshold varies around the country, but for London it is three consecutive days of 28C (82F) temperatures.

Meanwhile experts warned that the drought is far from over as the country needs weeks of rainfall to replenish supplies.
It comes after the country's driest July on record and the driest first half of the year since 1976.