Temperature passes 40C for first time; London fire brigade warns of ‘huge surge in fires’ – as it happened

London Heathrow records hottest ever temperature; London mayor Sadiq Khan says major incident has been declared

• Day of 40C shocks scientists as UK heat record absolutely obliterated'

Met Office chief meteorologist Neil Armstrong has a helpful explanation of what the chances are of the UK reaching 40C today and what this would represent:

Even higher maximum temperatures will develop tomorrow with a 70% chance of somewhere in England exceeding 40C. A value of this level would exceed the current UK record by 1.3C or more. This is akin to a marathon runner shaving 20 minutes off of the current record.

Weather forecast models are run numerous times to help us quantify the likelihood of a particular event occurring and estimate the uncertainty which is always present in weather forecasting to some degree. Some models are now producing a 70% chance of maximum temperatures in excess of 40C in isolated parts of the UK for the start of next week. Mid- to high-30s Celsius will be seen more widely with a 95% chance we will exceed the current record.

Climate change has already influenced the likelihood of temperature extremes in the UK. The chances of seeing 40C days in the UK could be as much as 10 times more likely in the current climate than under a natural climate unaffected by human influence. The likelihood of exceeding 40C anywhere in the UK in a given year has also been rapidly increasing, and, even with current pledges on emissions reductions, such extremes could be taking place every 15 years in the climate of 2100.