The yellow warning is in force from 9am until 23.59pm on Thursday and households are being advised to prepare for possible flooding in advance to protect their property, including turning off gas, water and electricity supplies.

The drastic advice given by the Met Office on how to prepare for the worst in the event of flooding also recommends preparing a flood kit, charging mobile phones and storing valuables up high to avoid being damaged.

In its advice for staying safe in heavy rain, the national forecaster said: “It’s never too late to take action and prepare for flooding. Follow these 6 simple steps to protect your home or business:

• Check the flood advice in your area to know when and where flooding will happen
• Charge mobile phone devices
• Park your car outside the flood zone
• Prepare a flood kit to help you cope in the event of flooding to your home and business
• Store valuables up high, including electrical devices, important documents and furniture
• Turn off gas, water and electricity supplies.”

You should only switch off your gas, water and electricity supplies if it is safe to do so and be sure that you don’t touch an electrical switch if you’re standing in water.

Your water stop tap will usually be found inside your home where your water supply comes in, or outside close to your property’s boundary under a cover, your gas shut-off valve next to your gas meter, and your mains electricity cut off on your fuse board with a big on and off switch.

The National Grid says: “If flood water is about to enter your property, turn off your electricity supply using the isolator switch or fuse unit. Only attempt this if it’s safe to do so, and never take any unnecessary risks. You can speak to your insurer or a qualified electrician if you need further advice. Your meter could be inside your property or outside in a dedicated meter cabinet.

“Never touch any electrical appliances, cables, or equipment while standing in floodwater. Please don’t touch any appliances, cables, or equipment that has been touched by floodwater, as they may not be safe.”

The 39 UK areas forecast to be affected by heavy rain on Thursday, with yellow weather warnings in place and a potential risk of flooding, are:

London & South East England

• Brighton and Hove
• East Sussex
• Hampshire
• Isle of Wight
• Oxfordshire
• Portsmouth
• Southampton
• Surrey
• West Berkshire
• West Sussex

South West England

• Bath and North East Somerset
• Bournemouth Christchurch and Poole
• Bristol
• Cornwall
• Devon
• Dorset
• Gloucestershire
• North Somerset
• Plymouth
• Somerset
• South Gloucestershire
• Swindon
• Torbay
• Wiltshire

Wales

• Blaenau Gwent
• Bridgend
• Caerphilly
• Cardiff
• Merthyr Tydfil
• Monmouthshire
• Neath Port Talbot
• Newport
• Powys
• Rhondda Cynon Taf
• Swansea
• Torfaen
• Vale of Glamorgan

West Midlands

• Herefordshire
• Worcestershire

Dan Holley, Met Office Deputy Chief Meteorologist, added: “A deepening area of low pressure will head north eastwards across the UK on Thursday, bringing heavy rain and potentially strong winds. The exact track of the low is uncertain, so it’s best to keep an eye on the forecast as the week progresses and we firm up on the details.

“A Yellow warning for rain covers southern England, southeast Wales and parts of the Midlands, with 20 to 40mm of rain expected widely. However, isolated spots could see up to 70mm, this most likely across parts of southwest England. Through Thursday evening and night, there’s also a chance of transient snow over higher ground hills in the north and west, as precipitation engages colder air on the northern and western flank of this area of low pressure.

“There’s the potential for some strong winds too, but this will be heavily influenced by the track and intensity of the area of low pressure. We are monitoring the low closely and advise staying up to date with warnings and the forecast as it develops.”