The Met Office reported a high of 28.7C (83.6F) at Northolt in west London around 4pm, beating the previous high of 28.6C seen in 1995.
It is also means today is the hottest May Day bank holiday Monday on record – beating the previous top temperature of 23.6C (74.5F), which was set in 1999.
The holiday was introduced in the UK in 1978.
Parts of the UK are enjoying temperatures hotter than holiday destinations including Madrid, Athens and Miami.
The South East and central southern England enjoyed the best of the sunshine, with most of the UK experiencing temperatures between 23C (73.4F) and 27C (80.6F).
Maximum average temperatures for May are 13C (55.4F) in the North and 16C (42.8F) in the South.
The record highest temperature for May is 32.8C (91F), which was recorded in 1922 and 1944.
However, not everyone has been basking under blue skies – it has been cloudier across Northern Ireland and Scotland, the Met Office said.
Low cloud, mist and sea fog is affecting the northern coast of Devon and Cornwall.
We are set to see cooler temperatures on Tuesday, with the weather picture becoming more mixed as the month progresses.