Roads have melted and train tracks have bent in the baking heat, as temperatures up to 31C are forecast for several more days.
Translink Rail to cancel services across Northern Ireland after rails reached temperatures of 49C.
Trains also had to be stopped from running over a section of track near Carlisle station and speed restrictions have been in place all week between London Waterloo to New Malden due to the scorching temperatures, causing delays.
Temperatures will remain high into the weekend and early next week, forecasters said.
Saturday could see 29C in England and Scotland, 27C in Northern Ireland and Wales and predicted highs of around 31C in some parts of the UK by Monday.
“We are going to see a continuation of the current warm spell but the temperatures over the next few days are not likely to be the warmest of the year so far," Met Office spokesman Grahame Madge said.
“I think for now temperatures have peaked and we are not expecting to see any records today. We will begin to have a slightly fresher feel, bringing the temperatures down, not by much, by one or two degrees.”
As road surfaces succumb to the heat, gritters have been deployed in some areas to spread crushed rock dust to create a non-stick layer between the surface and vehicles’ tyres.
Motorists have been advised to use warm soapy water to wash sticky tar from their vehicles.
Images of melted roads were shared by the staff of the Communication Workers Union as a warning to try and keep members of the public safe.
One, taken by a post office worker on Exmoor, which showed the current state of the road from Heasley Mill, near North Molton, Devon.
Up to eight heatwaves are forecast in July and August with temperatures set to hit 34C. Tarmac becomes soft at 50C, and while the air temperature is much lower, the black roads can become that hot in direct sunlight.
Social and health workers are on alert after the Met Office imposed a heat-health warning.
This week is the first time since 2013 that all four nations in the UK have seen temperatures of 30C or above, the Met Office said.
Public Health England issued a warning saying the extreme heat may pose a risk to the most vulnerable. The rising temperatures have also come with warnings to be careful near water and in the countryside, where fire crews have been tackling wildfires.
The family of 13-year-old Ryan Evans also warned of the dangers of playing and swimming in open water, after his body was recovered from Westport Lake in Stoke-on-Trent,
“We just cannot emphasis enough the dangers of swimming in open water, especially where young children are concerned," they said. “Our message is simple, please don’t do it.”
A 17-year-old boy was found dead in the River Aire in Leeds following a search and rescue operation. It is believed he got into difficulty while swimming on Tuesday evening.
The RNLI has urged those heading to the seaside to seek out beaches with a lifeguard.
There is bad news for farmers growing crops and the firefighters and soldiers battling the ongoing incident on Saddleworth Moor in north-west England as no significant rainfall is forecast in the coming days.
Mr Madge said: “There is a low probability of showers on Sunday. If they do bring some rainfall it would be in the south-west but that will be very sporadic.”
Parts of Saddleworth Moor have been alight for six days, with firefighters saying it could take weeks to extinguish.
A new moorland blaze – on Winter Hill, near Bolton Lancashire – broke out on Thursday with huge plumes of smoke visible from the area surrounding the moorland.
A 22-year-old man from Bolton has been arrested in connection with the blaze.
Firefighters are also still fighting a 500m-long gorse fire on the Glenshane Pass in County Londonderry, more than 48 hours after it first broke out.
The RSPCA said pet owners be mindful of their welfare, after it received 625 calls about animals in hot environments between 11 and 24 June – mostly regarding dogs in hot cars.
The hot spell is good news for gardeners, with horticultural experts saying it could help produce excellent vegetables, fruit and flowers.
The Royal Horticultural Society have suggested people use waste water from the kitchen to keep plants nourished and to reduce demand for water.
Additional reporting by PA and SWNS
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