A rare red weather warning for extreme heat has been issued for parts of England and Wales later this week as temperatures could reach 40C in places.
The Met Office warning affects London, parts of the Midlands, south-east Wales and southern England, and will be in place from 09:00 BST on Wednesday through to 21:00 BST on Thursday.
Forecasters say the extreme heat will be exacerbated by high humidity and warn that "significant disruption" to daily life is likely, including travel disruption. Some schools have also already announced plans to close early.
The Met Office said the hot weather will also have a population-wide impact on health and will lead to serious illness and a danger to life.
Less severe amber weather warnings are also in place across much of southern and central England and most of Wales from Monday through to the end of Thursday.
In the worst-affected areas, the Met Office has forecast that temperatures could reach up to 40C with the peak of the heatwave anticipated to fall on Wednesday and Thursday.
That would come close to the UK's highest temperature on record of 40.3C, set in Coningsby in July 2022. That is also the only previous time a red extreme heat weather warning has been issued.
Temperatures have risen quickly on Monday with a chance of thundery showers for some.
On Monday evening, the Met Office issued a yellow warning for thunderstorms covering parts of London, south-east and south-west England as well as the east of the country. It will remain in place until 21:00 BST.
Highs of 37C are then forecast for southern England and 35C in south-east Wales on Tuesday.
Temperatures are then due to rise to at least 39C on Wednesday and Thursday with the possibility this will be exceeded.
Across the hot spell, some areas will see temperatures not fall below 21C or 22C, resulting in a so-called tropical night.
The UK Health Security Agency (UKHSA) has issued its own red heat health warnings - marking the second-ever time it has issued such a warning related to heat.
The UKHSA warnings cover the West Midlands, East Midlands, South East, South West, London, and East of England and will be in place for a slightly longer time frame from 01:00 BST on Wednesday until 23:00 BST on Thursday.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, has urged health and social care services to ensure they are prepared for the coming days.
"It is vitally important that people understand the risk posed by high temperatures like these, and take steps to keep themselves and their friends, families and neighbours safe," he said.
As temperatures began to climb over the weekend and into Monday, some disruption is already becoming evident as the country looks to prepare for the days ahead.
The government does not normally advise schools to close during hot weather, but it suggests a number of measures to help manage the conditions.
A number of schools across England have taken measures including relaxing uniform rules and closing early over the coming days.
The National Association of Head Teachers (NAHT) said it has written to its members with guidance.
Paul Whiteman, the organisation's general secretary, said: "While there is no legal 'upper limit' for temperature in schools, (school leaders) will certainly be doing all they can to mitigate the effects of such high temperatures."
Meanwhile, the National Fire Chiefs Council (NFCC) has asked people to take extra care around open water and in outdoor spaces after a number of water-related deaths during the recent heatwave in May.
Chair Phil Garrigan said: "We are also asking people to be mindful of the impact their actions can have outdoors. Many people do not realise how easily a wildfire can start or how quickly it can spread."
South East Water has asked people to only use water for essential purposes as its network was under intense pressure.
Travel disruption is also very likely as temperatures increase, including to both railways and roads.
On the roads, the AA has said there will be gritters out on some roads to help in case surfaces soften or become sticky in the heat.
The organisation's president Edmund King advised drivers to slow down and avoid harsh braking or steering if they come across a softened or damaged road surface.
Rod Dennis, road safety spokesperson at RAC, has asked drivers in areas covered by red warnings to avoid non-essential journeys if their vehicles do not have effective air conditioning.
He warned that heat can put strain on vehicles and increases the potential for vehicle breakdowns.
Some Great Western Railway trains were delayed or cancelled on Monday due to the hot weather, and forecasters have warned disruption may become more widespread across the country's network as the week progresses.
Avanti West Coast is running fewer trains on some routes from Tuesday through to Thursday while Transport for Wales cancelled some train services on Monday and warned of further disruption in the coming days.
The heat is spreading from continental Europe where an extremely intense hot spell has developed.
Two children were found dead in a vehicle in France on Monday in an incident the local state prosecutor said was "probably linked to the heatwave".
Red warnings are in force in more than half of France's regions and across other parts of western and central Europe.
