Britain could be baked by the summer's fourth heatwave this week, as Southern Europe faces a "Molotov cocktail" of volcanic-hot 42C and wildfires fears. As deadly heat tears across Europe, half of France was under a heatwave warning while Spain's weather agency Aemet warned of "extreme danger" - with both forecasting 40C-plus temperatures.

Cristina Santín Nuño, a fire scientist at the Spanish National Research Council, said a wet spring helped plants grow then extreme heat and strong winds dried them out, adding: "If we add to this the relatively easy possibility that a spark can ignite a fire somewhere … we have all the ingredients for the ‘molotov cocktail’ we’re seeing right now." Now swathes of the UK are braced for our fourth heatwave - after two in June and one in July - with the mercury set to peak in the mid-30s in London on Tuesday.

On Monday, the UK Health Security Agency (UKHSA) issued an amber heat-health alert (HHA) for 5 regions of England - the West Midlands, East Midlands, South East, London, and East of England from 9am Tuesday until 6pm Wednesday.

In addition, a yellow HHA has been issued for the North West and North East regions for the same period while the South West and Yorkshire and the Humber regions are already under one.

Dr Paul Coleman, Consultant in Public Health at UKHSA, said: "Temperatures are forecast to rise above 30C across central and southern parts of the country over the next few days, and generally hot weather is expected across most regions of England.

"These kinds of temperatures can result in serious health outcomes across the population – particularly in those who are vulnerable, such as the elderly or those with serious health conditions - so it is important that everyone takes sensible precautions while enjoying the sun.

"If you have friends, family or neighbours who are vulnerable, it is important to ensure they are aware of the forecasts and are following the necessary advice. Check in on them if you can to make sure they know that hot weather is on the way and how to keep themselves safe."

The Met Office warns there is also a risk of isolated thunderstorms and infrequent outbreaks of rain alongside the high heat.

Greg Dewhurst, a senior meteorologist at the Met Office, said the temperatures over coming days mean that some parts of the country will meet heatwave criteria – three consecutive days exceeding a certain threshold – as early as Tuesday.

He said: "Temperatures are going to rise over the coming days, particularly across England and Wales, but it is not going to be felt quite as much across parts of Northern Ireland and Scotland – it’s not going to be widespread heat everywhere.

"And the reason for that is we’ve got high pressure to the south east of the UK at the moment and low pressure to the north west and west.

"The heat may spark one or two heavy showers and thunderstorms across south-west England and Wales, but they’ll be very isolated.

"Tuesday looks largely dry, with plenty of sunny spells – it could well be the peak of the heat. Temperatures are set to rise to 33C across central and southern England but this could spark off a thunderstorm by the afternoon.

"And some places will hit heatwave thresholds on Tuesday, while some places will hit it on Wednesday – it depends on the area."

Forecasters at the Met Office said that, in addition to high daytime heat, warm nights are also likely in south-eastern parts of the UK.

They add that there is a chance of a tropical night in a few places, where temperatures do not drop below 20C overnight.

Mr Dewhurst said: "Into Wednesday, we just start to see the high pressure edge away a bit.

"It allows the low pressure to the west to edge in a bit, so with a bit more cloud, temperatures may just be a bit lower, but we could still see it as high as the low 30s.

"On Thursday, temperatures are set to peak around 29C, again with a bit more cloud around and a few showers, before a drier Friday with sunny spells ahead of the weekend."

As wildfires rage across parts of the UK, including a major blaze on Dorset heathland and a fire at Edinburgh’s Arthur’s Seat, the Royal Society for the Prevention of Accidents (RoSPA) is urging the public to take hot weather risks seriously.

Over the weekend, more than 100 firefighters battled a wildfire in Dorset that forced the evacuation of 20 homes - while flames engulfed parts of Arthur’s Seat in Edinburgh, a popular beauty spot and tourist destination.

Steve Cole, director of policy at RoSPA, said: "These fires are a stark reminder that the UK is not immune to the dangers of extreme heat. With temperatures soaring and vegetation drying out, the risk of wildfires increases dramatically—even in urban and coastal areas.

"The fact is that many wildfires are preventable. Simple actions—like properly disposing of cigarettes, and avoiding open flames—can make a huge difference.

"Climate change is contributing to longer, hotter summers and increasing the frequency of extreme weather events in the UK. The recent fires underscore the need for greater public awareness and preparedness."