Temperatures could reach 35C today, making it the hottest day in almost a decade – and one of the worst commutes.
The heatwave has sparked speed restrictions on train lines across England amid fears tracks could buckle in the “extremely high temperatures”.
The precaution will make journeys in stifling carriages even longer on routes operated by First Great Western, Southeastern, Virgin Trains East Coast and Abellio Greater Anglia.
Meanwhile, parts of the London Underground are expected to be hotter than the legal temperature for transporting cattle.
As the mercury started to rise in this morning's rush hour, commuters took to Twitter to let off steam...
If I had wanted to be roasted alive and surrounded by wankers, I'd have booked a week in Magaluf. I'm being served it for free on the tube.— Rob Smith (@RobsterSM) July 1, 2015
New commute game - fainter roulette. Odds on the rather large chap that's been sweating profusely from Balcombe.— Matt Fletcher (@Fletch042) July 1, 2015
7:49 in the morning and I feel like I've stepped into an oven.. commute— JessOfLoveliness (@JessOfLovelines) July 1, 2015
Loving the new Sauna feature on the London Underground today— Daniel (@Daniel19Green) July 1, 2015
If you've ever wanted to experience hell, simply take a trip on the tube in the summer time.— Cas/Audio (@Cas_Audio) June 30, 2015
Some people really loved it though...
25c waiting for the tube in London at 7:30am this morning. This is stunning. Don't anytime say it's too hot, this is perfect.— James Bromley (@jamesbromley) July 1, 2015
Never fallen in love with so many women on the commute to work ☀ #SummerInTheCity— Owen Guest (@OwenGuest1207) July 1, 2015
Obligatory boat commute picture pic.twitter.com/v2b5mlLNlm— Allan Williams (@AllanDLWilliams) July 1, 2015
Employers have been urged to allow staff to travel outside rush hour to avoid overcrowded public transport during the exceptional temperatures.
Dr Angie Bone, head of extreme events at PHE, said the heat could be dangerous for older people, young children and those with serious illnesses.
Northolt in London saw the highest UK mainland temperature at 30.5C yesterday, while the Channel Island of Jersey hit 33C.
The highest July temperature on record was at Wisley, Surrey, in 2006, where the mercury peaked at 36.5C.
Thunderstorms could form in the Midlands and the North today before conditions cool slightly across the UK tomorrow.Reuse content