The UK has recorded its warmest November day, the Met Office said.
Temperatures soared to 22.3C (72.1F) in Trawsgoed, Wales - hotter than Barcelona and the Algarve.
The UK's previous record temperature in November was in 1946, when the mercury reached 21.7C (71.1F) in Prestatyn, Wales.
Meteorologist Nicola Willis said an "unusually warm air mass" for the time of year had settled over the UK.
She said: "Southerly winds brought warm air in from the continent and the settled conditions resulted in sunshine to that part of Wales."
The average maximum November temperature is 13C in the south of England, 10 to 12C across the rest of England and Wales and 10C in Scotland, Ms Willis said.
There was a 20% chance that the record could be broken again on Monday when more warm air is forecast, she added.
"I would not be surprised if we saw a repeat performance," Ms Willis said.
The warm temperatures came as thick fog caused more than 50 flights to be cancelled at Heathrow airport.