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

• UK reaches hottest ever temperature as 40.2C recorded at Heathrow

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 40°C. A value of this level would exceed the current UK record by 1.3°C 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 40°C 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 40°C 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 40°C 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.