Pessimists will say it was inevitable. After days of scorching heat, the UK is set to succumb to severe storms, which will rain down on much of the country over the next few days.

In a week of extreme conditions, the Met Office has issued a severe weather warning, predicting flash floods, lightning and high winds across much of the south and east of the country. The warning, which also forecasts "large hail" and perhaps even tornadoes, comes as the Government’s heatwave health alert remained in place.

A 16-year-old boy was today recovering in hospital after he was hit by lightening in Croydon, south London. He suffered burns to his body and an ambulance spokesman said he had had "a very lucky escape".

While similar conditions have already hit northern and western areas, causing roads to be closed and homes to be evacuated, the fresh wave of stormy weather is expected to cast its net wider. "The earlier storms were very much localised and came about as a result of high temperatures in the particular areas which were affected," said a spokeswoman for the Met Office. "These new storms are different - they stem from a weather front coming up from Spain - and look likely to spread across a wider area."

But those wanting to to cool off in the rain will be disappointed. While temperatures will drop slightly, into the high 20s for much of the UK, the storms will increase humidity levels, making conditions even more unpleasant.

"It’s going to be sticky and oppressive, with levels of up to 80 per cent humidity," said the spokeswoman.

With smog warnings already issued by the Government thanks to high pollution levels and the hot weather, health officials fear that the tumultuous weather could pose a further threat to those with breathing difficulties. Researchers say that people allergic to pollen who are in the path of a thunderstorm are likely to inhale air which is heavily loaded with pollen allergen and are consequently likely to experience an asthma attack.

During one weekend last June there was a sixfold rise in the number of emergency admissions for asthma due to thunderstorms.

Professor Martyn Partridge, Asthma UK’s Chief Medical Adviser, said: "We warn those with asthma or a tendency to asthma to make sure that they keep their medicines with them at all times over the next 24 hours. It is believed that the inversion of temperatures, that occurs at the time of thunderstorms, brings down pollen granules which the moisture then breaks up to a size that is breathed in to the lungs of those with asthma in large quantities, inducing attacks even in those who have just previously had mild asthma.

The heat has already led to health concerns, with the Government earlier this week raising its heat alert to level 3, signalling that vulnerable people in care homes should be moved to cooler parts of the building, and those living at home should receive daily visits.

NHS Direct said that it had been receiving 2,500 calls a day on average, a significant portion of whom were elderly people and those presenting symptoms of heat exhaustion or heat stroke. Local councils have been issuing their own warnings, with some producing their own guides to cope in the heat.

Meanwhile, people across the country have been devising their own enterprising methods of keeping cool. At the Arlington Reservoir in Sussex, ramblers watched as a nude cyclist rode his mountain bike through the water, while barristers and High Court judges were given permission to take off their wigs during court proceedings earlier in the week.

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