An Iberian plume left the capital scorching in temperatures warmer than Ibiza on Saturday.
Millions of Londoners basked in the hottest day of the year as the mercury soared to a record 30C.
However, it all goes downhill from tomorrow when thunderstorms are predicted to spark flooding, lightning, gales and possible power cuts.
A yellow thunderstorm warning has been issued for Wales and a large part of southern and central England from 2pm until 9pm on Saturday.
A second yellow warning for thunder is in place for Sunday from midday to 9pm for Wales, Northern Ireland and most of western and central England and Scotland.
Temperatures have reached above 30 degrees Celsius for the first time since 24th August 2022 making today the warmest day of the year so far 🌡️ pic.twitter.com/QyL6ixIJaR
— Met Office (@metoffice) June 10, 2023
Parts of east and northern Scotland are likely to avoid the most extreme conditions.
Dan Stroud, meteorologist for the Met Office, said: "On Sunday there will be a little bit more cloud, temperatures down a smidgen, but still 28C or 29C, sparking off a few showers and thunderstorm warnings.
"Early next week will be a continuation of hot weather. Monday there will be thunder, showers, but then the weather will try and settle down on Tuesday and mid-week.
"But it will maintain this well-above-average temperature. Most of the UK will meet heatwave criteria."
It came as least three soldiers collapsed during the Trooping the Colour rehearsal due to the sweltering heatwave.
The UK Health Security Agency (UKHSA) has issued an amber alert for hot weather until 9am on Tuesday 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.