Temperatures in the UK have reached 30C for the first time this year.
A temperature of 30.5C was recorded at Heathrow on Saturday, the Met Office confirmed in a tweet.
Warnings for thunder are still in place for later in the day and Sunday.
The Prince of Wales reviewed his troops on horseback on Saturday as at least three soldiers fainted in sweltering London temperatures.
Thousands gathered at Horse Guards Parade in St James’s Park to witness The Colonel’s Review ahead of Trooping the Colour during the King’s Birthday Parade next week.
The ceremony involved hundreds of horses and soldiers carrying out complex battlefield drill manoeuvres to military music.
But with temperatures reaching 28C, the baking heat proved too much for some, and the prince later tweeted his thanks to those who had taken part.
At least three soldiers fainted and were taken off on stretchers while two more were helped off the parade ground.
One member of the Massed Bands of the Household Division returned to his feet after collapsing and was greeted with a round of applause from the audience.
He appeared to attempt to carry on playing his instrument but was swiftly escorted off by medical staff.
William acknowledged the “difficult conditions” in a tweet he personally signed with his initial.
He wrote: “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.”
James Calford, 18, from Cardiff, who was the youngest soldier in the Welsh Guards to take part, described Trooping the Colour as “like being stood in a sauna with a 200kg dumbbell in your left hand”.
He said the ceremony “is a lot harder than it looks on the TV”.
“Once you have got your tunic and bearskin on and carrying your rifle in heat it is incredibly uncomfortable.”
The Colonel’s Review is the final evaluation of the Household Division before the soldiers and horses parade for the King during Trooping the Colour on June 17 to mark the monarch’s official birthday.
William inspected his troops on horseback, wearing a bearskin cap and red dress uniform, for the first time as Colonel of the Welsh Guards.Thunderstorm warnings are in place across much of the UK as temperatures soar across the weekend.
Warm air from the south is expected to send the heat rising, but bringing with it thundery showers and the chance of hail and gusty winds, according to the Met Office.
Chief meteorologist Frank Saunders said temperatures could reach 31C in parts of central and south east England on Saturday – along with the chance of torrential rain.
“As the heat builds from the south, thundery showers will develop through Saturday afternoon,” he said. “While not everyone in the warning area will see the heaviest showers, or even any rain at all, some will bring heavy thundery downpours.
“With intense showers there is a risk of surface water flooding which could cause some disruption.”
A yellow thunderstorm warning – highlighting potential disruption to travel, power cuts and localised flooding – has been issued for Wales and a large part of southern and central England from 2pm to 9pm on Saturday.
A second yellow warning is in place for midday to 9pm for Wales, Northern Ireland and most of western and central England and Scotland. Parts of east and northern Scotland are likely to avoid the most extreme conditions.
Mr Saunders said: “Sunday will see 30C again in parts of southern UK and the risk of thunderstorms more widely – particularly western and central areas.”
Forecaster Tomasz Schafernaker said the high temperatures will last well into the evenings, but are far from rare at this time of year.
“It is not unusual to get these sort of temperatures in June,” he said. “We’ve regularly approached the low to mid-thirties.
“We will notice the humidity rising as we go through the weekend with some really balmy nights on the way. At 10pm on Saturday, temperatures in some towns and cities will still be hovering around 20C.”
He continued: “For Sunday there’s a lot of sunshine around, very warm, very humid but with a greater chance of catching showers and thunderstorms across many western parts of the UK… hail, thunder and gusty winds and then it is back to sunshine again.”
The UK Health Security Agency (UKHSA) has issued an amber alert for hot weather until 9am on Monday 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”.
A further yellow alert, which is less serious and warns that there may be some disruption to services due to weather conditions, is in place for the North East, North West and Yorkshire and Humber regions of England, as well as London.
The Met Office said thundery showers may continue into Monday with conditions remaining hot and humid with plenty of sunny spells until midweek.
