Heavily-dressed ceremonial guards were withdrawn from their positions for a number of hours in the sweltering heat as temperatures topped 40C.
Soldiers, who wear a uniform including bearskin hats, were allowed to shelter from the sun in the hottest period on Tuesday.
The Ministry of Defence said the guards stationed at the Tower of London, Buckingham Palace, St James’s Palace and Windsor Castle were withdrawn at midday “given the extreme temperature” but remained on duty in the Royal Guardrooms to act in support of police if required.
Members of the Household Cavalry used electric fans to help them cope with the baking temperatures in London.
Troops were spotted looking red in the face and sweating while standing watch in their heavy ceremonial uniforms and helmets at Horse Guards Parade in Westminster.
One could be seen stood in position with a large metal desk fan pointed towards him in a bid to keep him cool.
Water was brought out to the horses of the mounted troops, and the animals were sponged down after guard duties.
An Army spokesman said: “The wellbeing of our soldiers is a priority at all times and we have put in place additional measures this week such as regular checks, flexible duties and additional water to ensure they can continue to safely carry out their duties.”
He said the Guard Mount at Buckingham Palace took place on Monday but with a shortened ceremony and guard personnel carried out reduced duties as temperatures peaked on Tuesday.