Temperatures are expected to climb again this week after a brief reprieve over the weekend.
Parts of southern England will yet again meet their heatwave criteria on Wednesday, which could reach highs of 32C (89.6F) or 33C (91.4F).
The rising temperatures may increase health risks, particularly for vulnerable groups, the UK Health Security Agency (UKHSA) has warned.
The existing yellow health alert for the East Midlands, East of England, South East and London has been upgraded to amber, running from 9am on Wednesday until 9am on Thursday.
A yellow alert will also come into force during the same period for the West Midlands, South West, Yorkshire and The Humber.
There are currently no alerts due to come into force for the North East and North West.
But 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.
Southern parts will “become hot” on Wednesday, but forecasters expect conditions to be “cooling” on Thursday with some isolated showers later in the week.
The UKHSA warned of possible “significant impacts” on health and social care services, including a rise in deaths among the elderly or people with health conditions.
There will also be “impacts on younger age groups” and the potential for more water-related incidents, including increased risks associated with “cold-water shock and drowning”.
Emergency services are also braced for further wildfires, while fire chiefs called for “everyone to play their part” amid an “exceptionally challenging summer” for firefighters.
A National Fire Chiefs Council spokesperson said: “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.
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The UK has had 20% of its average July rainfall, which takes into account the wetter areas.
It is “very, very unusual” for a hot and dry spell to last this long, Mr Petagna added.
The dry conditions come after last year’s drought summer, with the UK facing more extreme weather fuelled by human-caused climate change.
Meanwhile, wildfires continue to burn in the Cairngorms National Park in Scotland, across the Rhinogydd mountains in North Wales and Blaenavon in South Wales.
The last week has also seen blazes on heathland on the Isle of Wight, woodland in the New Forest and fields in Tadley in Hampshire.
The third heatwave in mid-July left firefighters battling 19 significant blazes simultaneously in England and Wales.
Europe wildfires: ‘Fire clouds’ spreading over Bordeaux
Across the UK – and Europe, the world’s fastest warming continent – people are facing the rising risk of extreme weather such as heat and drought.
Raging forest fires have forced more than 360,000 people to evacuate in France and Spain.
