Image caption Many people sought refuge at the seaside over the weekend

The heatwave currently hitting many parts of the UK is set to continue for at least another week, forecasters say.

The BBC weather centre said conditions would become more unsettled in northern areas towards the end of July, with the south enjoying warm weather for longer.

On Saturday, the highest temperature in the UK this year, 31.4C (88.5F), was recorded at Heathrow.

There are currently delays at London Waterloo train station because the heat has caused a rail to buckle.

Network Rail, which introduced a speed limit near the station last week due to high trackside temperatures, has closed platforms one to four at Waterloo and said "engineers are on site".

A reduced timetable is in operation and delays are expected to last for the rest of the day, with some trains cancelled and some London-bound services expected to terminate before reaching Waterloo.

On Sunday, motorists faced disruption when a section of the M25 was closed after the road melted.

It was shut close to junction 23, near Potters Bar in Hertfordshire, on the clockwise carriageway due to severe surface damage.

The Highways Agency said it reopened at 05:30 BST on Monday after emergency resurfacing.

"It is regrettable that drivers were stuck in traffic on such a hot Sunday afternoon, but the precaution was taken to close the carriageway after a defect across three lanes led to safety concerns," a spokesperson said.

'Baking conditions'

Monday is another warm and sunny day across much of the UK, with maximum temperatures of 30C, although parts of Scotland and Northern Ireland are seeing some cloud and patchy rain.

Media playback is unsupported on your device Media caption Heatwave Health Warning

BBC forecaster Nick Miller said high pressure was still dominating and the "baking conditions" would persist in many areas until the middle of next week.

The UK has been basking in above-average temperatures for the past week.

How to stay safe in the sun • Try to keep out of the sun between 11:00 and 15:00 • Wear sunscreen • Wear loose cotton clothing • Drink lots of cool drinks • Seek shade • Wear sunglasses and a hat • Look out for others, especially vulnerable groups such as the elderly, young children and babies, and those with serious illnesses • Never leave anyone in a closed, parked vehicle, especially infants, young children or animals Met Office weather warnings Age UK - how to stay cool during a heatwave NHS heatwave - be prepared

Wales saw its highest 2013 temperature of 30.2C at Llysdinam on Saturday. The highest temperature recorded for Northern Ireland was 29.9C on 8 July at Edenfel, County Tyrone. Scotland saw its highest 2013 temperature on 9 July, with 28.7C at Strathallen Airfield.

The Met Office has issued alerts under its Heat-Health Watch scheme for the East Midlands, East of England, south-east England and London.

The "yellow" alerts - level two out of four - mean there is a 60% chance of heatwave conditions in these areas over the next few days.

Met Office heatwave thresholds are based on maximum daytime temperatures - which vary by region from 28C in north-east England to 32C in London - and a minimum night temperature of 15C.

In other developments:

In Scotland, a wildfire has been blazing south of Inverness since Sunday afternoon. About 500 acres of bracken is on fire between Laggan and Dalwhinnie in Strathspey, and 25 firefighters are using beaters, while a helicopter is dropping water.

The hot weather has prompted Rabbi Alan Kimche, one of London's leading orthodox rabbis, to warn his congregation about dehydration during a fast, which is due to begin on Monday night for 25 hours

Music fans who attended Scotland's T in the Park festival over the weekend celebrated in temperatures as high as 26C. More than 8,000 sachets of sunscreen were used, 700,000 litres of water consumed and 104,500 ice creams eaten. There was a rise in the number of medical visits from 810 in 2012 to 1,160 this year, largely as a result of the weather

In addition to festival-goers, sunbathers, sport-watchers and barbecue fans have also welcomed the hot weather.

Retailers have reported booming sales as the heat saw shoppers spend thousands of pounds on the high street and online on barbecues, food, sunscreen and garden furniture.

Image caption Punters enjoy the summer sunshine as they make their way along the river Cam in Cambridge

Asda said sales of barbecues had soared by 204% in the last two weeks, with charcoal and fuel sales up 176%.

The supermarket also said shoppers were keen to cool off, with paddling pool sales up 446% and sales of garden furniture up 44% as Britons indulged in al fresco dining.

But many shoppers avoided stressful and sweaty trips to the shops by turning to the internet.

Online retailer Amazon said that, compared with this time last year, sales had increased by 816% for paddling pools; 519% for sprinklers and sprayers; 543% for patio chairs and loungers; and 145% for sun skincare products.

But the hot weather is not good news for everyone. The very young, the elderly and the seriously ill are the groups who are particularly at risk of health problems when the weather is very hot.

Image caption The good weather has been around for a while - this was the scene at a Northumberland beach a week ago

In particular, very hot weather can make heart and breathing problems worse.

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

It advises those who are vulnerable to the effects of heat not to go out in the sun between 11:00 and 15:00 - the hottest part of the day. It also recommends that people drink cold drinks regularly.

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