It has been the hottest late August Bank Holiday Monday ever, as temperatures soared across the UK.
Temperatures had reached 33.2C (91.8F) at Heathrow by 14:16 BST, the Met Office said, beating the previous record of 28.2C set two years ago.
Trawsgoed near Aberystwyth saw the top temperature of 24.3C in Wales, while Scotland's highest was 24C at Fyvie Castle in Aberdeenshire.
In Northern Ireland, thermometers peaked at 21.2C in Killowen.
On Sunday, the record for the hottest late August Bank Holiday weekend was broken, with a high of 33.3C.
Last month, the UK's highest ever temperature was officially recorded in Cambridge at 38.7C.
It beat the previous UK record of 38.5C, set in Kent in 2003.
Meanwhile, in Frinton-on-Sea, Essex, beachgoers hoping to enjoy the sunshine were left struggling to breathe after what some witnesses were told might have been a fuel spill.
Police, paramedics and the fire service attended the beach at Fourth Avenue after receiving calls at about 14:00 BST on Sunday.
The cause is unknown and is being investigated.
Miriam Lansdell told the PA news agency: "My daughter started coughing. She said 'I don't feel good. It hurts to breathe in'."
The government's advisory Committee on Climate Change has warned the UK is not prepared for the increase in heatwaves that is expected with global warming.
Earlier this summer, it called for more action to make sure homes and other buildings such as hospitals and care facilities could be comfortable and safe in higher temperatures.
Meanwhile, the London Ambulance Service advised people attending Notting Hill Carnival on Monday to drink plenty of water, use sun cream and remain in the shade where possible.