A reading of 34C (93.2F) was taken by the Met Office at Heathrow and in west London.
A new record temperature has been set for 2019, with the mercury expected to keep rising.
The Met Office said readings of 34C (93.2F) were taken at Heathrow and elsewhere in west London on Saturday afternoon.
It said this was just short of the 35.6C (96F) June record set in 1976.
Both Northolt and Heathrow have reached 34.0 °C today, not only making it the #hottestdayoftheyear but one of the warmest June days for the UK in around 40 years