As Britain braces for the hottest days of the year so far, doctors are urging people to know the danger signs of heat stroke – a medical emergency that can turn fatal within hours.

Triggered when the body overheats and can no longer cool itself down, heat stroke can lead to seizures, organ failure, coma and death if not treated quickly.

The warning comes as a blistering heatwave grips the UK, with temperatures forecast to climb towards the high 30s in parts of England and Wales this week.

The Met Office has issued extreme heat warnings, while health officials have warned that soaring temperatures can pose a risk not only to older and vulnerable people, but also to otherwise healthy adults and children if they become too hot or dehydrated.

Last summer, there were an estimated 1,504 heat-associated deaths in England during five heat episodes, according to UK Health Security Agency (UKHSA) figures. 

Dr Ann Nainan, a GP specialising in urgent care, said: 'Heat stroke is life-threatening and needs life-saving treatment.

'We all need to be able to recognise the early symptoms and take corrective action immediately.'

So how can you tell the difference between feeling hot and tired – and a potentially deadly emergency? Here, experts reveal the warning signs, what to do if someone is affected, and the simple steps that could help keep you safe during the heatwave.

The difference between heat stroke and heat exhaustion

There are two different types of heat stroke; one is brought on as a result of physical activity in extreme heat, while the other is brought on from gradual exposure to the high temperatures. 

Heat exhaustion can come before heat stroke and should be treated quickly so it doesn't progress. 

Common symptoms of heat exhaustion include developing a high temperature, feeling extremely thirsty due to dehydration, and becoming irritable or confused.

People suffering from heat exhaustion may also feel tired, dizzy, have a headache, feel nauseous or like they want to vomit. Excessive sweating, pale or clammy skin, and muscle cramps in the arms, legs or stomach are also signs.

To prevent heat exhaustion from progressing to the more dangerous heat stroke, the NHS advises moving the affected person to a cool place as soon as possible. 

Any unnecessary clothing, such as jackets or socks, should be removed, and they should be encouraged to drink plenty of water or rehydration drinks containing electrolytes.

Cooling the skin can also help. This can be done by spraying or sponging the person with cool water, applying ice packs, or placing a cold, damp cloth on their skin.

If symptoms do not improve within 30 minutes, emergency medical help should be sought immediately, as the condition may have progressed to heat stroke.

Dr Nainan told The Telegraph, 'the sooner you cool down, the less damage will be done'.

How does heat affect the body after 5-10 minutes?

Large parts of the UK are set to feel temperatures above 35C today - and experts say just five minutes in these conditions can have impacts on the body.

According to Dr Nainan, the body will start working harder almost immediately to maintain its natural temperature of around 37.3C. 

As temperatures rise, heart rate increases, as does sweat production - in an attempt to lower the body's temperature.

This process can lead to fluid loss and dehydration, particularly if a person is exposed to direct sunlight or not drinking enough water. 

At this early stage, experts recommend taking simple preventative measures to reduce the risk of heat-related illness. 

Wearing loose, lightweight clothing and applying sunscreen can help protect against the sun's harmful ultraviolet (UV) rays, while drinking water regularly is essential to replace fluids lost through sweating.  

If no action is taken, however, discomfort can set in after just 10 minutes - with sweat becoming more intense, and dehydration beginning to set in. Headaches can even be felt at this point. 

The good news is that this can be prevented with just a few minutes in the shade and a drink of cool water. 

What about after 10-40 minutes?

If no preventative measures are taken, the effects of being in extreme heat can be felt more intensely after just 10 to 20 minutes.

At this point the body will be using large energy resources to cool down. Feelings of thirst and skin burning - if no sunscreen has been applied - will also start becoming more noticeable.

By around 40 minutes, experts warn that many people could begin experiencing the early stages of heat exhaustion.

Dr Nainan warns that people may begin feeling symptoms such as cramps and spasms due to a lack of electrolytes in the body.

Other warning signs, at this stage, include headaches, nausea, dizziness and persistent sweating.

Without taking action - such as moving into the shade, finding a cool space and rehydrating - the risk of developing heat stroke is real.

After one hour 

By spending an hour exposed to the extreme heat, experts warn that the body's temperature can rise beyond its normal level to 38.5C - officially known as hyperthermia.

This can bring on heat exhaustion, which can - but not always - turn into heat stroke.

At this point, Dr Rowley Cottingham, a director of the British Association for Immediate Care and a consultant in emergency medicine, says people will begin to 'feel incredibly tired and have an overwhelming desire to stop everything'.

It is now crucial to prevent the condition from worsening.

If taking shaded or cool shelter is unavailable, soaking clothing with water can help lower body temperature, while taking small, frequent sips of water can help with rehydration. 

Eating foods rich in electrolytes - such as bananas, watermelon and salty snacks - can also help replace minerals lost through intense sweating.

If symptoms persist despite these steps, or begin to worsen, medical advice should be sought immediately to prevent heat stroke.

Between 90 minutes and two hours  

If no preventative measures are taken - or they are not effective - the body will now enter a state of heat stroke, according to experts.

Having failed to cool itself down, the body's temperature can rise to 40C, which can trigger brain, heart and liver malfunction - and lead to death within just minutes. 

Dr Cottingham says: 'It's incredibly dangerous: it kills very quickly. You're literally cooking like an egg. Body proteins suffer heat damage and this happens to every organ.'

The brain is first affected after swelling in the heat - which is often why heat stroke sufferers feel confusion. This can be followed by sufferers struggling with balance and coordination, while their skin often becomes hot and dry to touch.

Sweating may also stop altogether as the body's cooling system breaks down. Other symptoms can include a rapid heartbeat, dizziness, weakness, nerve dysfunction and an increased risk of fainting. 

As heat stroke takes hold, the consequences become increasingly severe. Loss of consciousness can occur, while the most serious cases may lead to seizures, coma and, ultimately, death if treatment is not provided urgently.