At last, the UK looks set to enjoy a real summer. Tuesday will be the hottest day of the year and the heatwave is forecast to last well into next week.

But there are two sides to sunny weather. While the population at large has been rejoicing around the barbecue, health authorities have warned that extreme summer heat is set to become the norm – and that we need to get better at preparing for it.

The deaths of two soldiers during a training exercise in Wales on Saturday have brought home the risks.

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A&E wards in some parts of the country had higher than average admissions over the weekend and figures released by Public Health England tomorrow are expected to show that calls to NHS Direct and 111 phone lines about heat-related concerns spiked last week.

The heat, UV rays, high pollen counts and air pollution all combine to create a major public health headache. Tuesday is expected to be the hottest day of the year, with the Met Office forecasting the temperature will pass 32C for the first time since last August.

After the heatwave of 2003, when temperatures topped 38C and there were 2,000 extra deaths over 10 baking days in August, public health authorities brought in a comprehensive heatwave plan. It predicts that by 2040, the extremes of temperature seen in 2003 will become the norm.

“This is everybody’s problem. We need to look after everyone and we need everyone to look out for each other,” said Professor Virginia Murray, head of health protection at Public Health England.

“After 2003 we realised we had a very vulnerable population and had to start looking at the consequences of extreme events.”

The elderly, the long-term ill and children are all at high risk. Those with cardiovascular and respiratory illnesses are at greatest risk. Dehydration is the main issue, but high temperatures are also linked with increased levels of potentially harmful pollutants.

At the moment, most areas of the UK are at a level-two heatwave alert. Care homes and hospitals are required to ensure cool zones are below 26C and that there is plenty of water and ice. The air quality index is at low to moderate risk – meaning it is safe to exercise.

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On Tuesday, some areas could enter a level-three heat alert, which demands close monitoring of patients in hospitals and care homes. A prolonged spell of temperatures in the high-20s and 30s, coupled with above average night-time temperatures would trigger a level-four alert, demanding an emergency response from the Government.

The hope is that long-term planning will mean Britain is better prepared than ever. During the 2003 heatwave in France, there were 15,000 extra deaths in only three weeks.

“The importance of staying cool in hot weather cannot be overestimated for older people, especially at night between successive hot days,” said Michelle Mitchell, charity director at Age UK. “It is extremely important older people take sensible precautions, particularly if they have breathing problems or a heart condition.”

“It’s not classic heat that kills,” added Prof Murray. “It’s people who are vulnerable and maybe cannot move themselves to a cooler room –people who may be socially isolated and therefore not able to recognise that they’re really not coping.”

Other vulnerable groups include the homeless, who are more likely to be exposed to high temperatures in inner city “heat islands”.

Public Health England would recommend that marathons and other running events should not go ahead during a level-three threat – temperatures above 30C for most areas.

Britain’s infrastructure is also beginning feel the heat. Train speeds have been reduced as rails have reached temperatures up to 52C, while a section of the M25 was closed on Sunday near Potters Bar in Hertfordshire.

Wildlife trusts around the country have reported a jump in the populations of a range of species, such as butterflies, dragonflies and bumblebees on land and jellyfish and leatherback turtles in the sea.

In other cases, the weather is allowing nature to make up for lost time. Conditions were so poor in the spring that many barn owls, swallows and other birds were unable to breed because there were so few insects and worms to eat.

Mark Champion, Wigan projects manager at the Lancashire Wildlife Trust, said: “The warm weather has created a second spring with birds just starting to nest.”

But he added: “We are worried ponds may dry up before froglets and toadlets are old enough to leave. It could be a problem unless we get some rain.”

Firefighters are urging people to take care with cigarette butts.

“A small spark from a cigarette is often all it takes to start a grass fire. Drivers also need to take care not to throw cigarettes out of car windows,” said Dave Brown, the London Fire Brigade’s head of operations.

London firefighters are dealing with 15 grass fires a day and that number could rise if the hot weather persists.

However, perhaps the most bizarre result of a hot spell is that some insects such as flower bugs and harlequin ladybird larvae turn their attentions from plants to humans.

Matt Shardlow, chief executive of Buglife, said: “You can get bugs trying to suck moisture out of people rather than plants. This doesn’t cause real harm, just momentary pain, but is an indication of how desperate they are.”