A new record temperature has been set for the late August bank holiday weekend, the Met Office said.
A temperature of 31.6C (88.9F) recorded at Heathrow on Sunday, which is just above the 31.5C record set at the same location in 2001.
The late summer sunshine, as a result of warm air being dragged up over the UK from France – comes at the end of what has been a wet and chilly month so far.
Temperatures are expected to rise even further by Monday, possibly reaching a late August bank holiday record high of 33C – most likely in the south-east of England, the Met Office said.
On Saturday, Gogerddan, near Aberystwyth, recorded 27.4C – a new record for the highest late August holiday weekend temperature in Wales.
The mercury rose to 30.7C at Heathrow, west London, just shy of the record of 31.5C set in the same location in 2001.
Aberdeenshire in Scotland could see 29C on Sunday, while Northern Ireland is likely to enjoy 24C.
People attending Notting Hill Carnival on Sunday and Monday – expected to number one million – have been advised to stay hydrated.
Carnivalgoers have been advised to be prepared for very hot weather by taking water or refillable bottles and downloading the official carnival app to keep up to date with the location of medical points and food stops, and to get updates on transport delays.
Forecaster Craig Snell said: “People certainly need to bring plenty of water with them and slap on the suncream.”
Anyone travelling over the weekend has been advised by the RAC to pack enough food and water, and plan enough breaks for the journey.
Saturday’s high in Wales beat the previous record of 27.3C (81.1F) at Velindre, Powys, in 2013.
The Northern Ireland record stands at 27C (80.6F) recorded in Knockaraven, Co Fermanagh, in 2003.
Monday is not a bank holiday in Scotland.