A soldier collapsed in the heat on Monday during a rehearsal for the state visit by the Emperor and Empress of Japan.
The soldier was pictured face down on the ground on the Mall before being helped to their feet by a fellow guard.
It is understood the cavalry guard was rehearsing for a procession to be held on Tuesday as part of the state visit being hosted by King Charles by emperor Naruhito and his wife.
The incident came as Londoners were heading to the parks and lidos and dusting off the barbecues as temperatures headed towards 30C.
After the UK’s cool start to the season, the capital was bathed in sunshine —with the hot spell set to last until at least Thursday.
A high of 27C was predicted for central London today, with the mercury hitting 29C tomorrow, 28C Wednesday and 27C Thursday during a mini-heatwave.
It could even hit 30C for the first time this year in parts of London and the South-East.
Liam Eslick, a meteorologist at the Met Office, said: “According to the latest models, there is a chance temperatures could reach 30C.
“It’s not guaranteed but this will likely be towards the South-East of England, particularly London, Essex and Kent.”
It marks a break from the rainy spring, which saw 32 per cent more rainfall than the average, making it the fifth wettest for England, the Met Office said.
However, a yellow heat health alert has been triggered for most of England including London.
The alert has been issued for every region except the North-East, with some areas being warned of the potential for “significant impacts” on the health and social care sector. The health alert from the UK Health Security Agency (UKHSA) and Met Office came into force at 8am and remains in place until Thursday afternoon.
The UKHSA warns that in London, there are likely to be “minor” impacts on health and social care, including an “increase in risk of mortality among vulnerable individuals and increased potential for indoor environments to become very warm”.
Mr Eslick said: “My advice to people would be by all means go out and enjoy. They have been waiting for summer.
“But UV levels are going to be high and the sun will be very intense. Wear protection like sun cream, stay hydrated and try to get some shade in the midday sun. People sensitive to the sun should take particular precautions.”
He added that the longer-term weather forecast remained uncertain, with a “breakdown” expected from late Wednesday into Thursday.
He said: “It could be a little thundery with a breakdown towards the back end of the week. But Monday, Tuesday and Wednesday are looking fine.”
An official UK “heatwave” occurs when a place records at least three consecutive days with daily maximum temperatures meeting or exceeding the threshold, which varies across the UK. In London, that temperature is 28C.
