The temperature in Wales is expected to reach 31C today.
Weather presenter Derek Brockway has said at that 11am, the temperature was already 28C in Cardiff with a high of 31C expected this afternoon. In Ceredigion, the temperatures are already 29C. Milford Haven has reached 26C already.
He said that the temperatures across Wales are "generally hotter than yesterday".
He added that there is already a prediction it could reach 34C in Bristol on Tuesday and 36C on Wednesday.
Emma Sharples, meteorologist at the Met Office, said that Tuesday is set to be slightly cooler with temperatures rising on Wednesday.
People are being warned to wear high factor sunscreen as there are strong UV levels.
Dermatologist Dr Rachel Abbott has advised people to seek shade between 11am and 3pm. She also advises to cover up and wear a hat.
The weather is set to peak on Wednesday and turning cooler and fresher from Thursday.
On Sunday, a high of 29.1C was recorded in Bute Park.
The highest temperature recorded in Wales in June was on June 18, 2000 where 33.7C was recorded in Machynlleth.
Why is it so hot?
In short - the warm weather is being caused by high pressure building from the south west and warm air being drawn up from the Azores - a group of islands in the mid-Atlantic near Portugal.
The UK is currently on the warmer side of the jet which will remain to north of us for much of the week, keeping temperatures above average.
This, mixed with the time of year, clear skies and high pressure has paved the way for the warm and sunny conditions.
But, the high pressure dominating our weather will eventually give way to a weak frontal system this week, bringing more changeable weather.
Ms Sharples said: "The weather we're experiencing at the moment is due to air being drawn up from warm locations across the Mid Atlantic.
"We also have hot weather across Spain and France and that air is leading up across us. That, mixed with clear skies and high pressure, means any cloud and rain is being pushed out."
At 11am today, Met Office measuring stations were already sweltering:
Usk: 27.5C
Swansea: 23.4
Milford Haven: 24.5
Rhyl: 24.4
Trawscoed: 27C
RAF Valley: 26.4C
Cardiff: 27.6C
