A national emergency could be declared across the country as temperatures soar towards 40C.
Government ministers are drawing up plans as a Level 4 Heatwave Alert could be announced, with the UK already set at level 3 for most of England and Wales for Sunday and Monday.
Level 4 means even fit and healthy people could be in danger of illness and death due to the extreme weather, not just those in vulnerable groups.
Headteachers could be left to decide whether to open or close schools as classrooms and common areas get too hot to be safe for youngsters and staff.
They look set to be given the same powers to choose what is right for their pupils as they do on a snow day.
The Government’s emergencies committee - COBRA - began a meeting at noon today to discuss the ongoing heatwave issues.
The Prime Minister's official spokesman said there were "tried and tested" plans in the NHS for increasing staffing in relevant areas.
He continued: "There have already been some co-ordination meetings at official level across Government.
"This is a progression of that to ensure that - as we come into what are predicted to be the hottest days - that the necessary mitigations and actions are in place or being but in place.
"A lot of this is making sure the public are aware of the advice that is available - particularly those with young children or vulnerable family members."
Asked what going further will look like, he replied :“It would depend on the duration and issues like that, in terms of what action the government or indeed local councils may choose to take."
It is unclear who will decide about what happens to local services such as bin collections and whether it will dictated at government level or left to local council leaders.
Officials are warning that people living in south-facing top-level flats, those with drug and alcohol dependencies, and the homeless are all at high risk from the extreme conditions.
They also warn that there could be an increase in the number of pet deaths due to irresponsible owners leaving them in restricted enclosures with poor ventilation.
The unusually hot weather is not good news for farmers as it will lead to damaged crops, spoiled produce as a result of hot storage conditions, and lower-than-normal yield of fruit, vegetables and grains.
Increased demand for water supplies could also see local hosepipe bans brought in by water companies.
Disruption is expected across the road and rail network, with users of the London Underground advised to carry water with them.
And most worryingly, the high temperatures can result in lower efficiency of power stations, with nuclear reactors at risk of tripping out above 40C.
Workers will however be expected to go to work as The Health and Safety Executive (HSE) says although employers have a responsibility to ensure that working conditions are "reasonable" there's no specific temperature limit.
They add that employers have a responsibility for "keeping the temperature at a comfortable level, sometimes known as thermal comfort".
The HSE says there is no limit on high temperatures due to some manufacturing roles requiring constant heat.
Mr Johnson's spokesman added: “There will be further coordination meetings throughout the week.
“It’s up to UKHSA [the UK Health Security Agency] and the Met [Office] to decide whether to move further.
“But as it stands there are health alerts for the majority of the country because we expect temperatures to remain high.
"Advice has been issued already both to the general public and the NHS.”
According to the official Heatwave Plan for England, the decision on whether to declare an emergency would be taken coordinated by the Cabinet Office.
A UKHSA spokesman said: "There's a possibility of a level four heatwave. If it gets above 40C, then it is likely to be a level four heatwave for the first time.
"I don't see how it couldn't be in those temperatures."
The Met Office has said that heatwaves are the deadliest extreme weather event in the UK with an average of 2,000 heat-related deaths each year.
Met Office deputy chief meteorologist Rebekah Sherwin said: "From Sunday and into Monday, temperatures are likely to be in excess of 35C in the South East, although the details still remain uncertain.
"Elsewhere, temperatures could be fairly widely above 32C in England and Wales, and in the mid-to-high 20s further north."
She added: "Weather forecast models are run hundreds of times to determine the most likely weather outcome.
"For late in next weekend and early next week, some runs of these models are allowing exceptionally high temperatures to develop, which is something we'll be monitoring closely and adding details in the coming days.
"Some models have been producing maximum temperatures in excess of 40C in parts of the UK over the coming weekend and beyond."
