Britain is set to get even HOTTER today as temperatures soar to sizzling 89.6F

The Met Office has issued its first ever extreme heat warning as parts of the UK are set to swelter in 91.4F temperatures until Thursday with further advice over travel chaos, power cuts and 'adverse health effects'.

Thunderstorms and showers are expected to hit the south of the country tonight and will stay for the next few days interspersed with more boiling weather.

Parts of Sussex have already seen heavy rain and thunderstorms taking place this afternoon as the Met Office has warned there could be up to 50mm of rainfall before 7pm tonight. 

The heat warning - which is similar to those issued when heavy rain or snow is forecast - covers a large part of Wales, all of south-west England and parts of southern and central England, and will remain in force until the end of Thursday.

It is the first time the warning has been issued since the Met Office said last month that it would launch weather warnings for extreme heat after a record-breaking number of heatwave deaths were recorded in England last summer.

Temperatures are expected to reach 91.4F in some western areas later this week, with mid to high 80s expected elsewhere.

Chief operational meteorologist at the Met Office, Steven Ramsdale, said: 'The high temperatures are going to continue through a large part of this week. Many areas will continue to reach heatwave thresholds but the amber extreme heat warning focuses on western areas where the most unusually high temperatures are likely to persist.

'There's a continuing risk of isolated thundery downpours late in the afternoons but most areas will stay dry until later in the week. Temperatures should begin to fall for most areas heading into the weekend, with some more unsettled conditions looking to develop.'  

Records were set in both England and Wales on Sunday, and the mercury is forecast to climb even higher on Monday.

In England on Sunday, 88.88F (31.6C) was recorded in Heathrow, overtaking Saturday's record-breaking 86.54F (30.3C) recorded in Coton in the Elms, Derbyshire. 

Three children were treated for sunburn across 'fairly large areas of their bodies' over the hottest weekend of the year in Britain. 

And five people died in separate drowning incidents across England as the country experienced the hottest weekend of the year so far. 

Emergency services are urging sunseekers and bathers to stay safe after the bodies of three teenagers, a man of an unknown age and a man in his 50s were pulled from rivers and lakes. 

Yesterday the mercury hit 88.9F (31.6C) in Heathrow, west London, and 86F (30.2C) in Cardiff.

And Swansea's Morriston hospital saw three children admitted to the Welsh centre for burns and plastic surgery over Friday and Saturday. 

The Swansea Bay NHS care board said: 'Our burns centre has been treating a lot of children with serious sunburn and staff fear a further surge in cases in the coming days. 

'Please remember to be sun safe. Just a few minutes applying a high-factor sun screen could save you and your child a lot of pain and suffering.' 

It comes after parents were urged to use sun cream on their children yesterday. 

The Met Office's warning states that people in the danger areas can expect:  

Adverse health effects are likely to be experienced by those vulnerable to extreme heat The wider population are likely to experience some adverse health effects including sunburn or heat exhaustion (dehydration, nausea, fatigue) and other heat related illnesses More people are likely to visit coastal areas, lakes and rivers leading to an increased risk of water safety incidents Some changes in working practices and daily routines likely to be required An increased chance that some heat-sensitive systems and equipment may fail, leading to power cuts and the loss of other services to some homes and businesses Some delays to road, rail and air travel are possible, with potential for welfare issues for those who experience prolonged delays. 

Doctors are calling on parents to protect their kids from the blazing sun as temperatures soar across Britain. 

A spokesman for the Welsh centre added: 'We have seen three children admitted in the last two days with sunburn to fairly large areas of their bodies.

'As you can imagine, it's quite painful and could have been avoided if they had applied protection.'

Dr Zoe Lee added: 'Our advice to parents is to encourage their children to play in the shade as much as possible, wear hats and long sleeves, and apply regular applications of sun cream to the skin. 

'It's also important to apply sun cream regularly and to keep hydrated.'