The UK's meteorological agency on Friday issued its first ever "red" warning for exceptional heat, forecasting record highs of 40 degrees Celsius next week.

"Exceptional, perhaps record-breaking temperatures are likely early next week," Met Office chief meteorologist Paul Gundersen said.

The Met Office said there was a 50-percent chance of temperatures topping 40°C (104 degrees Fahrenheit) for the first time on Monday or Tuesday, and an 80-percent chance that the country's previous record of 38.7°C set in 2019 will be exceeded.

"Nights are also likely to be exceptionally warm, especially in urban areas," said Gundersen.

"This is likely to lead to widespread impacts on people and infrastructure. This level of heat can have adverse health effects."

A red warning is issued when it is "very likely that there will be a risk to life, with substantial disruption to travel, energy supplies and possibly widespread damage to property and infrastructure".

The warning will cover next Monday and Tuesday for parts of central, northern, eastern and southeastern England.

The service said that while "extreme heat events do occur within natural climate variation", their increase in recent decades "is clearly linked to the observed warming of the planet and can be attributed to human activity". 

UK hospitals have already warned of a surge in heat-related admissions and train operators have told passengers to expect cancellations.