"Temperatures at Heathrow have recently reached 33.5 degrees, which tentatively breaks the record for May," the Met Office said on X.
The previous record for May was 32.8 degrees Celsius. It was recorded twice, in 1922 and in 1944.
Monday's 33.5 degrees in Britain was hotter than both Madrid and Athens. It was also the hottest bank holiday on record, Sky News notes. A bank holiday is a public holiday when most people in the country are off work.
It is also unusually hot in France. Parts of Brittany have been under a yellow heat warning since Sunday.
A spokesperson for France's weather agency Météo France told AFP it was the first time a yellow heat warning had been issued in May since the system was created in 2004.
In the capital Paris, 31.9 degrees were recorded on Saturday.
