A temperature of 38.7C was recorded in the UK on Thursday, appearing to break the record for the hottest ever day.
The recording at Cambridge Botanic Gardens, which the Met Office says is provisional and requires verification, would outstrip the previous record high of 38.5C, recorded in 2003.
Forecasters previously confirmed Thursday was the hottest July day on record, with a temperature of 38.1C, also recorded in Cambridge, surpassing the previous high of 36.7C set at Heathrow Airport in 2015.
It was also the second hottest UK day on record, beating the 37.1C recorded in August 1990.
Dr Mark McCarthy from the National Climate Centre (NCIC) at the Met Office said: "As the official source of meteorological statistics for the UK, we take the quality of our recordings very seriously.
"We are talking about a potential new record for the highest temperature recorded in the UK and we therefore need to thoroughly investigate the observation with our partners from Cambridge University Botanic Garden through statistical analysis and by visiting, to check the site and equipment and ensure there are no potential problems."
The high temperatures were driven by hot air funnelled from the south as western Europe was gripped by an extreme heatwave.
Also on Thursday, the mercury hit its highest ever level in Belgium (41.8C), Germany (41.5C) and the Netherlands (40.7C), in each case breaking a record that was only set on Wednesday. Paris saw a record high temperature of 42.6C.
Met Office experts say there is "no doubt" climate change is playing a role in driving the unprecedented temperature highs.
The soaring UK temperatures were followed by thunderstorms which swept across the country overnight.
On Friday morning they were continuing to bring heavy rain, gusty winds and lightning to the south-east of England.
"The thunderstorms could continue on and off through the course of the day," Met Office meteorologist Greg Dewhurst said.
"There is enough energy in the atmosphere to keep it going."
Temperatures will range from the mid-20s to the low 30s on Friday, Mr Dewhurst added.
The country will see more familiar conditions return over the weekend, with the weather becoming less settled and an increased chance of rain for many eastern and north-eastern areas.
