People are being advised to stay out of the sun at the hottest time of the day, drink plenty of water or fruit juice, and use factor 15 plus sunscreen.

If you do go out try to stay in the shade and wear a hat and light, loose-fitting clothes.

You can stay cool indoors by keeping curtains closed in sunny rooms and taking cool showers or baths.

Those most at risk of heatstroke – which can be fatal – are babies, young children and the elderly.

Symptoms of heatstroke include headache, nausea, an intense thirst, sleepiness, hot red and dry skin, a sudden rise in temperature, confusion, aggression, convulsions and loss of consciousness.

If you suspect someone has heatstroke dial 999 immediately.

Dr Richard Garlick, public health consultant at NHS Hertfordshire, said: "I want people to enjoy the sunshine but to do it safely.

"If you follow this common sense approach you can do both."

For more information visit www.nhsdirect.nhs.uk