At least three troops have fainted in the scorching heat during a royal military parade on Saturday - with Prince William acknowledging the "difficult conditions".
It happened as temperatures exceeded 30C (86F) in the UK for the first time this year - 30.5C was recorded at Heathrow and 31.2C was later recorded in Surrey.
According to the Met Office, the 30C mark is the earliest it has been reached in England in nearly two decades.
The Prince of Wales, who led the Colonel's Review at Horse Guards Parade in central London, paid tribute to soldiers on social media.
In a tweet, he said: "A big thank you to every solider (sic) who took part in the Colonel's Review this morning in the heat. Difficult conditions but you all did a really good job. Thank you. W."
Photos of the troops showed one being taken away on a stretcher by medics.
During the Colonel's Review, there were reportedly more than 1,400 soldiers and 200 horses from the Household Division of Welsh, Irish, Scottish, Grenadier and Coldstream Guards paraded for inspection by Prince William.
It was held ahead of the Trooping the Colour parade, which takes place next Saturday.
The ceremony involved horses and soldiers carrying out complex battlefield drill manoeuvres to military music.
Ahead of the hot weather, the Met Office issued its first heat-alert across much of England.
The UK Health Security Agency (UKHSA) issued an amber heat-health alert for hot weather until 9am on Tuesday in the West Midlands, East Midlands, East of England, South East and South West.
It means the impact of the heat is "likely to be felt across the whole health service… and the wider population, not just the most vulnerable".
At about 11am on Saturday, Kew Gardens, Heathrow, East Sussex and Surrey saw temperatures peak around 30C.
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Sky News weather producer Joanna Robinson said the heat and humidity will continue over the next few days.
"Temperatures continue to climb this afternoon, with a current high of 31.2C at Chertsey in Surrey," she said.
"It's not unusual to reach the 30C mark in June, in fact all dates except the 13th June have done so since records began.
"But according to the Met Office, it's the earliest that England has reached it in nearly two decades, but for the UK as a whole it's been since 2012."
Robinson said it took the UK an "unusually long time" to reach 25C this year.
She added: "We could see 32C somewhere in central or eastern England, but that's still a way off the highest temperature recorded for June, which was 35.6C back in 1957 and 1976."
Despite the continued warm weather, some parts of the country are also facing torrential rain, hail and gusty winds, with a yellow thunderstorm warning out for some parts of the UK, including Wales and a large part of southern and central England.