Three members of the military passed out during the Colonel's Review this morning as royal fans gathered to watch Prince William stepping up for his first major role during Trooping the Colour.
The Prince of Wales carried out the Colonel's Review this morning, a week before the King's Birthday Parade next week.
With temperatures reaching 30C in London, least three troops appeared to faint in the hot weather during the Colonel's Review at Horse Guards Parade, with William acknowledging the "difficult conditions".
Nowadays, five of the British Army's foot guard regiments wear bearskin hats and ceremonial uniforms during events such as Trooping the Colour.
Heavy heat this weekend has prompted the UK Health Security Agency (UKHSA) to issue an amber warning in eastern and southern England.
The alert, in force until 9am on Monday, says high temperatures could affect all ages and impact the health service.
The Met Office also issued a yellow warning for thunder across all Wales and large parts of southern England from 2pm until 9pm today.
Today's ceremony, led by William, was the final review of the parade before the main event attended by the King and other senior royals during Trooping the Colour on June 17.
The first review was carried out last week by Major General Christopher John Ghika CBE.
Over 1,400 soldiers of the Household Division and The King's Troop Royal Horse Artillery are taking part in the review, including 400 musicians from the Massed Bands.
Next week's ceremony will also include 250 soldiers from the 1st Battalion Grenadier Guards, the 1st Battalion Coldstream Guards, the 1st Battalion Irish Guards and, for the first time, the 1st Battalion London Guards, who will line the processional route along The Mall.
Just after 9am this morning, the Prince of Wales, Colonel of the Welsh Guards, who will be inspecting the troops, tweeted: "Months of preparation and training go into Trooping the Colour.
"Today members of the Household Division will perform their final major rehearsal for the event in front of members of the public in London."
After the ceremony, 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. Thank you. W"
Trooping the Colour, also referred to as the King's Birthday Parade, is a celebration of the monarch's birthday and takes place in June.
It is a display of pageantry involving numerous troops and after the ceremony, the monarch joins other members of the Royal Family on the balcony of Buckingham Palace for a fly-past by the Royal Air Force.
The first Trooping the Colour which is reviewed by The Major General usually takes place two weeks before the actual Birthday Parade.
The second Trooping the Colour which is reviewed by The Colonel of the Regiment usually takes place a week before the actual Birthday Parade.
Last year, during the first review by The Major General a woman fell through a stand alongside an eight-year-old boy and crowds were evacuated by emergency services.
