At least 10 people were taken to hospital with “heat-related conditions” after a major crash on the M25 caused queues of up to eight hours.
With London and the south east suffering through a red weather warning, several drivers were left stranded after the crash between a van and a lorry on Wednesday.
South East Coast Ambulance Service (Secamb) said it had responded to calls at around 10am on Wednesday, and a section of the motorway between junctions six and seven was closed as a man was airlifted to hospital.
Secamb said that the congestion was "severe and prolonged", with vehicles stuck for hours on end on the hottest June day the UK has ever seen.
The service confirmed that they received a number of calls from people suffering from heat-related illnesses, including a number of coach passengers, and that its crews had assessed and treated patients, taking 10 to hospital.
Many people took to social media to vent their frustration, with some asking for help from the emergency services.
Folkestone and Hythe MP Tony Vaughan pleaded with the police, writing: "Please sort this out urgently. People need water and some may need evacuation. You have protocols for this situation - please implement them - this is an emergency!"
He added that one woman had been stuck on the M25 near Oxted for eight hours.
He said: "It is 40 degrees in her car. She has kids with her. She is running out of water. The person from the car in front has been airlifted to hospital with dehydration. Police seem to be saying it's not their job to give out water."
One woman wrote on Facebook: "My parents have been stuck on the M25 for over 4.5 hours, no information, no supplies.
"There are elderly people, babies, animals, people with health conditions out there on the tarmac in the middle of a red heat warning on the hottest June day for 50 years!
"Can the police not coordinate some water and assistance? The exits are taking hours!"
Another woman said she had been stuck for five hours and had to turn her air conditioning off to save petrol, adding she "felt very wobbly".
A provisional temperature of 36.1C was recorded in Gosport, Hampshire, according to the Met Office, making it the hottest June day on record in the UK.
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A rare red weather warning for extreme heat remains in place.
Surrey Police confirmed that the crash involved a white Mercedes Vito van and a white lorry, with the van driver, a man in his 30s, suffering a leg injury and receiving further treatment in hospital.
A force spokesperson said: "One of the vehicles is currently being recovered, and a significant oil spillage from the collision has caused damage to the road surface.
"National Highways engineers are carrying out assessments before the carriageway can safely reopen."