During a royal military parade on Saturday, at least three troops fainted in the heat, and Prince William acknowledged the "difficult conditions."
The Met Office later confirmed that 30.5 °C was recorded at Heathrow, marking the first time this year that temperatures in the UK exceeded 30 °C (86 °F).
William tweeted following the event: “A big thank you to every soldier who took part in the Colonel's Review this morning in the heat. Difficult conditions, but you all did a really good job.”
The event was a practice run for Trooping the Colour, a military parade that is held every year in June to celebrate the monarch's official birthday. On June 17, the event will be overseen by King Charles III.
More than 1,400 soldiers and 200 horses from the Household Division of the Welsh, Irish, Scottish, Grenadier, and Coldstream Guards were reportedly paraded during the Colonel's Review for Prince William's inspection.
It was held in front of the Marching the Variety march, which happens next Saturday.
Horses and soldiers performed intricate battlefield drill maneuvers to military music during the ceremony.
The Met Office issued its first heat alert for much of England ahead of the scorching weekend.
The UK Health Security Agency (UKHSA) issued an amber heat-health alert for the West Midlands, East Midlands, East of England, South East, and South West until 9 a.m. on Tuesday due to the heat.
Accordingly, "likely to be felt across the whole health service… and the wider population, not just the most vulnerable," the heat will have an effect.
Temperatures reached 30 °C in Kew Gardens, Heathrow, East Sussex, and Surrey around 11 a.m. on Saturday.
Forecasters said it could be the hottest day of 2023 so far, but some areas may also have torrential rain, hail, and strong winds.
The Met Office has issued a yellow thunderstorm warning for some parts of the UK this afternoon, so it's not all sunshine.