It has been the hottest late August bank holiday weekend ever – with temperatures reaching 33.3C (91.94F) in west London.
The Met Office confirmed the new record temperature, which was set at Heathrow.
It beats the previous record of 31.5C (88.7F), set in 2001 and again at Heathrow.
The hot temperatures are expected to continue to rise this afternoon, forecasters say.
On Saturday a new record for Wales was set in Gogerddan, where 28.8C was recorded.
In Scotland, which does not have a late August bank holiday, temperatures were slightly cooler – but still reached 27C in Auchincruive, Ayrshire.
BBC Weather said temperatures around the UK on Monday would be similar to those on Sunday.
The south-east of England could experience temperatures of between 32C and 33C, which would be a national record for the late August bank holiday Monday, it added.
The London Ambulance Service advised people attending Notting Hill Carnival on both days to drink plenty of water, use sun cream and remain in the shade where possible.
Last month, the UK’s highest ever temperature was officially recorded in Cambridge when 38.7C was measured.
It beat the previous UK record of 38.5C, set in Kent in 2003.
The government’s advisory Committee on Climate Change has warned the UK is not prepared for the increase in heatwaves that is expected with global warming.
Earlier this summer, it called for more action to make sure homes and other buildings such as hospitals and care facilities could be comfortable and safe in higher temperatures.