A high of 29.5C (85.1F) is recorded in west London as people flock to enjoy the warm weather outdoors.
The UK has recorded its hottest day of the year so far, with temperatures set to rise on Friday, the Met Office said.
Northolt, west London, saw temperatures reach 29.5C (85.1F), surpassing the previous hottest day of the year on Wednesday which saw 28.2C (82.8F).
On Friday the mercury is expected to rise to 34C in the South East, exceeding temperatures in parts of Jamaica and the Maldives.
Across most of England and Wales highs of between 27C and 30C are expected.
The Met Office said the “unusual” temperatures for June is a result of high pressure over the southern half of the UK and a south-westerly airflow bringing warm air across the UK and Europe.
The weather has not yet beaten the record for the hottest June day ever, which was a high of 35.6 C at Southampton Mayflower Park in June 1976.
It comes after the UK Health Security Agency (UKHSA) and Met Office issued a level three heat-health alert for London, the south-east and east of England, with the alert remaining in place until midnight on Saturday.
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.”
A cold front is expected to push across the country from the north through the weekend, bringing a return to average temperatures, although high temperatures are expected to remain in East Sussex and Kent with 29C possible.
Dr Mark McCarthy, head of the Met Office’s National Climate Information Centre, said climate change had increased the average temperatures of UK summers and the likelihood of experiencing more extreme temperatures during hot spells and heatwaves.
He said: “Reaching 34°C 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.”