Media playback is unsupported on your device Media caption BBC weather presenter Chris Fawkes: "It is difficult to keep yourself cool and your core temperature can rise, causing some pretty big problems."

Health warnings have been issued as heatwave conditions are reached in London and the South East.

The Met Office issued a "Level 3" warning on the hottest day of the year, with temperatures hitting 32C (89.6F) at Northolt, west London.

The warning alerts healthcare services to help those in high-risk groups such as the elderly and young children.

Two similar heatwave warnings were issued last week in Yorkshire and the Humber, and in south-west England.

It is the UK's first prolonged heatwave since 2006.

While the warm weather has been welcomed by sunbathers, sport watchers and barbecue fans, health officials said the heat could be dangerous for very young children, elderly people, pregnant women and those with serious illnesses.

How to stay safe in the sun Hot weather poses a real danger to health. The very elderly and the seriously ill are most at risk but people at the peak of physical fitness can still succumb, particularly if they are under extreme physical exertion. The main threat is dehydration - as we overheat we can soon lose more fluid than we take in. The body can no longer cool itself and our core temperature, which should be a stable 37C, rises. At 40C the cells inside the body begin to break down and the body starts to malfunction. It even stops being able to sweat, leading to more overheating. The heart rate and breathing rate speed up and the person may fit, hallucinate or become unconscious. Heatstroke is a medical emergency that needs urgent treatment. Met Office weather warnings BBC Weather Centre Age UK - how to stay cool during a heatwave NHS heatwave - be prepared

The heat has also caused problems for drivers and rail passengers in parts of England this week, after road surfaces melted and tracks buckled in the heat.

In other developments:

Police in Norfolk are warning of the dangers of swimming in open water. It comes after the bodies of a man and a teenage boy were recovered from a water-filled quarry near King's Lynn, Norfolk

Police in Newcastle say lives are being put at risk by vandals breaking fire hydrants and spraying themselves with water to cool down

The heatwave has brought a 15% rise in the demand for tap water in London and the Thames Valley. Thames Water's nine million customers have been using about 400m litres a day in addition to the 2.6bn litres they usually get through

Water UK, which represents all major water companies, says there are no plans to impose a hosepipe ban as reservoir levels are where they should be

Firefighters in London have dealt with twice as many grass fires compared with last year. There have been 1,010 incidents so far this summer

Doctors in Wales say the number of people suffering sunstroke, sunburn and heatwave-related injuries is stretching hospital emergency departments

Some MPs want bosses to send staff home if workplaces get hotter than 30C. The proposal is contained in a parliamentary early-day motion. There is no suggested maximum limit in the current rules

Level 3 is one notch below the most serious warning in the Met Office's heat-health watch system.

The temperatures that trigger local action plans vary according to location and may involve health, housing, and social care services, to protect those susceptible to the heat.

In north-east England it is 28C, in Wales 30C and in London 32C. The night-time trigger temperatures vary from 15C to 18C.

Image caption One youngster combined a dip with some derring-do Image caption Others have taken the opportunity to enjoy an outdoor swim Image caption The heatwave has left some people sweltering in front of their fans Image caption Dedicated fishermen are not to be put off by soaring temperatures Image caption But rail passengers have faced delays as tracks buckle in the heat Image caption Norfolk police warned of the dangers of quarry lakes, after two swimmers died Image caption Londoners have cooled off in the capital's fountains Image caption The butterfly population has exploded in the warm weather previous slide next slide

Alerts are triggered when threshold temperatures have been reached for one day and the following night, and the forecast for the next day has a greater than 90% confidence level that the day threshold temperature will be met.

Scotland and Northern Ireland are not included in the alert system.

BBC weather forecaster Chris Fawkes said a heatwave, according to the World Meteorological Organization, occurred when temperatures were five or more degrees above average for at least five days.

He said the UK was already in a heatwave and the alert was essentially a health warning, telling healthcare professionals to watch out for vulnerable people.

While most places will be largely dry and very warm on Wednesday, northern and western parts of the UK will be cloudy.

Health officials in England are advising people experiencing the very hot weather to stay cool, drink lots of cold fluids and keep an eye on those they know to be at risk.

Dr Angie Bone, who is leading Public Health England's heatwave plan, said its efforts involved health and social care workers in the community; hospitals and care homes regularly checking on vulnerable patients; sharing sun safety messages; and making sure room temperatures were set below 26C.

Under the plan, officials ensured patients had access to cold water and ice, and that medicines were stored in a cool place, she added.

The NHS says the main risks posed by a heatwave are dehydration, overheating, heat exhaustion and heatstroke.

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