Amber heat health alerts are now in force across parts of the country as a fourth heatwave hits the UK, with forecasters warning of a “North-South split”.

The UK Health Security Agency (UKHSA) warned rising temperatures may increase health risks, particularly for vulnerable groups, as the East Midlands, London, the South East and east England were placed under amber heat health alerts from 9am on Wednesday until Thursday morning.

Yorkshire and the Humber, the West Midlands and south-west England are under yellow heat health alerts for the same period.

In recent days, temperatures have cooled, winds have picked up and a scattering of rain has fallen for some, but the heat is expected to spike again in the South this week.

Parts of southern England will yet again meet their heatwave criteria on Wednesday, which could reach highs of 34C. Others will breach the threshold on Thursday.

The UKHSA said: “The existing yellow heat-health alert for the East Midlands, East of England, South East and London has been upgraded to amber and will be in place from 9am on Wednesday 29 July until 9am on Thursday 30 July.

“A yellow heat-health alert will remain in place for Yorkshire and the Humber, West Midlands and South West regions for the same time period.

“There is no alert in place for the North East and North West regions.”

Amber heat health alerts could see a rise in deaths among vulnerable groups such as the elderly, as well as an increased demand on health and social care services. With internal temperatures set to rise, care settings like hospitals and care homes may exceed the recommended threshold for clinical risk assessment and workforces may struggle.

People may struggle to manage medicines and the power grid could exceed capacity. The health agency also warned that water-related incidents may increase, including risks from cold water shock and drowning.

Met Office deputy chief forecaster Gregory Wolverson said that while this would be a short-lived spell of heat, temperatures would climb quickly throughout the first half of the week, especially in the South East.

“By Wednesday, some areas could see temperatures in the mid-30Cs, and while the heat will not be widespread across the whole of the UK, it will be enough for parts of the South East to likely meet official heatwave criteria by Thursday,” he added.

"Elsewhere, the weather will be more changeable, and the change to cooler, more unsettled conditions will arrive sooner from the north and west, with a mix of sunshine and showers. Across northern areas, it is possible there will be heavier showers with a small risk of isolated thunderstorms through the week.”

Forecasters are expecting this week’s hot spell to be a “North-South split”, with clouds and outbreaks of rain continuing in Northern Ireland, Scotland and northern England.

But the peak will be “almost a blink and you miss it”, with temperatures widely cooling close to average by the weekend, said senior Met Office meteorologist Dan Stroud.

July has been 3C above average across the UK, according to provisional Met Office data on the month so far.

This could put it among the hottest three or four months of July on record, said Mr Stroud.

The dry spell has contributed to low water levels, and North Wales and the Upper Severn became the first part of the UK to declare drought status on Thursday.

Emergency services are braced for further wildfires during the next heatwave.

At the height of the third heatwave, in mid-July, firefighters were battling 19 significant wildfires simultaneously in England and Wales.

Meanwhile, blazes have wrought destruction in mainland Europe this week, with 220,000 forced out of their homes in south-west France, and authorities in Spain have evacuated 790,000 while 30,000 have been confined to their homes.

This comes as a Met Office meteorologist said that UK heatwave thresholds will need to be adjusted to account for the warming climate, which has seen temperature records repeatedly broken this year.

A National Fire Chiefs Council (NFCC) spokesperson said: “With the Met Office warning that a fourth heatwave could develop this week, bringing another period of very hot weather to parts of the UK, fire and rescue services remain on alert and are continuing to prepare for renewed wildfire risk.”

Battling nearly 20 wildfires at once demonstrates “the scale of the challenge posed by the prolonged hot, dry conditions”, they added.

“Most wildfires are preventable and we are asking everyone to play their part in helping to protect our communities, wildlife and natural spaces, while easing the pressure on firefighters who have already faced an exceptionally challenging summer,” they said.

The London Fire Brigade said it had attended more than 1,300 incidents, including grass fires and a person rescued from the Thames, over the weekend as the hot weather continued.

The service said it handled 2,729 emergency calls between Friday and Sunday, and firefighters attended 1,345 incidents across the capital, including a grass fire which destroyed 10 acres in Rainham, a hectare of grassland alight in Croydon, and blazes at flats, homes, sheds, a bakery and a storage unit.

The Brigade’s fire boat crews also rescued a person in the Thames near Battersea Pier.

The fire service is warning people not to use disposable barbecues in parks, to dispose of smoking materials responsibly, take litter home or put it in the bin, and use barbecues at home carefully away from anything that could catch fire, like sheds.

Fires, if they start, are likely to become exceptionally severe on Wednesday in pockets of the East, South, and south-east England, according to Met Office data used by Natural England and Natural Resources Wales.