The UK's Meteorological (Met) Office on Friday issued a first-ever red extreme heat warning for parts of England, including London, next week as temperatures have continued to soar in the last few days.

A red warning, the highest used by the Met Office under its alert system, means a risk to life is likely as temperatures could hit 40 degrees Celsius. The unprecedented warning implies there is a danger to life across all age groups and is classed as a national emergency.

"We've just issued a red warning for extreme heat for Monday and Tuesday, which is the first such warning ever issued," said Met Office spokesperson Grahame Madge.

"The warning covers an area from London up to Manchester and then up to the Vale of York...If we get to 40 degrees Celsius, that's a very iconic threshold and shows that climate change is with us now. This is made much more likely because of climate change, he said.

The Met Office warns "adverse health effects" may be experienced by many and will not be limited to those most vulnerable to extreme heat. Met Office chief executive Penny Endersby described the extreme heat forecast as "absolutely unprecedented" and urged the public to take warnings seriously.

"Our lifestyles and our infrastructure are not adapted to what is coming. Please treat the warnings we are putting out as seriously as you would a red or amber warning from us for wind or snow, and follow the advice," said Endersby.

"Stay out of the sun, keep your home cool, think about adjusting your plans for the warning period," she said.

The UK's national weather service also warned there is a high risk of failure of heat-sensitive systems and equipment, potentially leading to localised loss of power and essential services, such as water or even mobile phone services. The public have also been warned to expect delays on roads, along with disruption to rail and air travel.

The UK Health Security Agency (UKHSA) also issued its highest heat alert at level four, which it says is reached when a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system. This means that fit and healthy people could be susceptible to illness and death and not just high-risk groups, UKHSA said.

The hot weather is being treated as a national emergency with contingency plans in place, Downing Street said.

For other parts of the United Kingdom, the already existing amber warning has been extended to the rest of England, Wales, and parts of Scotland.