Several British soldiers were overcome by the heat on Saturday as they turned out in woolen tunics and bearskin hats to salute Prince William.
At least three guardsmen fainted during the military parade known as the Colonel's Review, in which more than 1,400 soldiers of the Household Division and the King's Troop Royal Horse Artillery were reviewed by the heir to the throne, who is honorary Colonel of the Welsh Guards.
The temperature in London on Saturday was due to hit 86 degrees Farenheit.
Afterwards William tweeted: "A big thank you to every solider who took part in the Colonel's Review this morning in the heat. Difficult conditions but you all did a really good job."
He later shared several photos of the parade, writing that the "hard work and preparation that goes into an event like this is a credit to all involved, especially in today's conditions."
The event was a rehearsal for Trooping the Colour, an annual military parade held each June to mark the monarch's official birthday. King Charles III will oversee the ceremony on June 17.
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