The Met Office said the provisional record, which still needs to be confirmed, was recorded at 12.50pm at London's Heathrow Airport, surpassing the previous high of 38.7C recorded in 2019.

Stephen Belcher at the Met Office said he had not expected to see such temperatures in the UK in his career.

"Research conducted here at the Met Office has demonstrated that it's virtually impossible for the UK to experience 40C in an undisrupted climate but climate change driven by greenhouse gases has made these extreme temperatures possible," he said.

With the mercury still rising, train routes from London up the east and west coast of the country were cancelled, electricity companies reported mass outages and normally busy city centres appeared quiet.

Network Rail tweeted a number of pictures showing large bends and kinks in rail tracks.

To the east of the capital a large fire engulfed homes in the village of Wennington, with flames tearing across neighbouring tinder-dry fields and approaching a historical church.

Elsewhere large grass areas around the capital were on fire, blowing smoke over major roads and nearby areas.

London Fire Brigade declared a major incident and urged people to stop having barbecues.

The UK, which can struggle to maintain key transport services in extreme heat or snow, had been put on a state of national emergency over the unprecedented temperatures.

Transport minister Grant Shapps said there had been a considerable amount of travel disruption.

"Infrastructure, much of which was built from the Victorian times, just wasn't built to withstand this type of temperature," he said.

Australian Associated Press