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The Met Office has issued heatwave alerts across the south and east of England, with temperatures expected to exceed 30C (86F) in some areas.

A level three heatwave alert is in place for the east of England, where high daytime temperatures and warm, humid nights are forecast until Friday.

London and south-east England are on a level two alert, meaning heatwave conditions are also likely this week.

Public Health England urged people to be aware of the health risks of heat.

To stay safe, people should drink plenty of water, wear loose-fitting cotton clothing, and stay out of the sun between 11:00 and 15:00 BST, it said.

Temperatures of 30C have already been recorded in Cambridge, Writtle in Essex, and Heathrow on Tuesday.

What is a heatwave?

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Although there is no official definition of a "heatwave" in the UK, the term can be used to describe an extended period of hot weather relative to the expected conditions in an area at that time of year.

This is different to the Met Office's four heatwave alert levels, which trigger action from health professionals and emergency planners.

The criteria vary by region - for example, the daytime temperature threshold in north-east England is 28C, but in London it is 32C. The average threshold temperature is 30C by day and 15C overnight for at least two consecutive days.

Parts of the UK are currently at level three of four - four being a "national emergency" red alert, which is issued when a prolonged hot spell becomes severe.

Source: Met Office

Public Health England (PHE) also urged people to look out for older people, those with underlying health conditions and young children who may be particularly vulnerable as temperatures rise.

It also advised people to:

Close curtains on rooms that face the sun to keep indoor spaces cooler - but remember it may be cooler outdoors than indoors

Drink plenty of water - sugary, alcoholic and caffeinated drinks can make you more dehydrated

Open windows when it feels cooler outside

Never leave anyone in a closed, parked vehicle - especially young children or animals

Walk in the shade, apply sunscreen and wear a hat when outside

Avoid physical exertion in the hottest parts of the day

People should also take care and follow local safety advice if going swimming in open water to cool down, PHE added.

Six people died around the UK and Channel Islands coastline in a series of incidents last weekend.

How to stay safe at the beach

Image copyright PA Image caption A canoeist made the most of the warm weather with a paddle on the River Thames near Sonning in Berkshire

Image copyright PA Image caption Beachgoers soaked up the sun on a baked beach at Lee-on-Solent in Hampshire

Dr Thom Waite, a consultant in public health medicine at PHE, said: "Each year we hear stories of people who have fallen seriously ill because, even though it's hotter, they may wear clothes which are too warm for hot weather, they may not drink enough or try to do too much."

He said as it was still the school holidays for many, and with a number of high-profile events planned for this weekend, people should "stay hydrated, look out for others and keep an eye on forecasts".

Wales and north England are also expected to experience warm temperatures this week, reaching highs of about 28C (78.8F) on Tuesday afternoon.

Image copyright Getty Images Image caption Holidaymakers were treated to blue skies on the North Wales coast at Llandudno

Image copyright Getty Images Image caption The barbeques were out in Henley-on-Thames in Oxfordshire

Met Office meteorologist Steve Willington said the warm and sunny conditions were expected to hold in the south-east of England until Friday.

However, Tuesday's high temperatures could give way to scattered thunderstorms and showers pushing up from the south-west across the West Midlands towards Humberside from Wednesday evening and into Thursday.

Scotland and Northern Ireland are forecast to experience lower temperatures throughout the week, with cloud and some showers possible.

More unsettled weather is expected for the bank holiday weekend - although the Met Office said there was a chance for a brief return to warmer conditions early in the weekend across at least southern parts of the UK.