The hottest day of the year so far has been recorded in St James Park, central London on Wednesday, when temperatures reached 28C.
Further highs of 34C (93.2F) are expected to arrive by Friday in some parts of England, prompting a Met Office expert to warn of a 'rare' climate change event.
The Met Office and the UK Health Security Agency (UKHSA) issued a level two heat health alert for a large part of southern and central England, with a level one alert in place for northern England.
Wednesday's 28C heat has beat the previous record for this year of 27.5C (81.5F) set in May at Heathrow airport.
A hot spell is set to build to a crescendo of 34C on Friday in the South East - making it warmer than Greek Islands Santorini, Mykonos and Zakynthos, as well as Los Angeles and parts of Barbados.
Announcing Wednesday as the hottest day of the year, the Met Office tweeted "expect that to be beaten again this week".
But the sizzling heat in the south is in stark contrast to the weather in northwest Scotland, where temperatures are struggling to push past the mid-teens.
Dr Mark McCarthy, the head of the Met Office National Climate Information Centre, warned climate change could be the reason behind the high temperatures.
He said: "Climate change has increased the average temperature of UK summers, and it is also increasing the likelihood of experiencing more extreme temperatures during hot spells and heatwaves.
"Reaching 34C during June is a rare, but not unprecedented, event in the historical climate records for the UK.
"But if it should happen this week it would be notable that it would have occurred on three days during the last six Junes."
He added the other days were 21 June 2017 and 29 June 2019.
Before that the only previous Junes when England saw a number of stations reach 34C or higher were 3 June 1947, 29 June 1957, and 26-28 June 1976.
Greg Dewhurst, a Met Office forecaster, added that the later part of this week is expected to be "very warm".
He said: "The vast majority of England and Wales will see a lovely day on Wednesday, though slightly different in Northern Ireland and Scotland, which will see cloud and showers.
"It will be very warm in the South East in particular, with the temperature hitting a high of 28C, which would make it the warmest day of the year so far.
"This will then be surpassed on Thursday, with the mercury rising again to around 29C before reaching 33C and even 34C on Friday.
"So it's going to be very warm as we move into the later part of the week."
A heatwave is defined as three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.
Agostinho Sousa, head of extreme events and health protection at the UKHSA, said: "During periods of hot weather it is especially important to keep checking on those who are most vulnerable, such as older people and those with heart or lung conditions.
"Make sure to look out for signs of heat exhaustion and follow our simple health advice to beat the heat."