ByBen Rich
Lead Weather Presenter
Saturday could provide momentary respite from the heat after sunshine lifted temperatures above 30C (86F) in parts of the UK on Friday.
Fresher air pushing in from the west should suppress temperatures - although highs of 27 or 28C (81-82F) are still expected in south-east England.
Most places should be dry with sunny spells and just a chance of an isolated shower.
However, temperatures are set to rise again on Sunday before climbing further on Monday and Tuesday as a Met Office amber warning of extreme heat takes effect.
Will a heatwave be declared on Saturday?
For a hot spell to officially become a heatwave, temperatures must exceed a threshold - which varies from 25-28C (77-82F) in different parts of the country - for three consecutive days.
There are parts of south-east England that have now recorded two days above this threshold so a third, on Saturday, would mean a heatwave being declared.
However it is possible that maximum temperatures may fall slightly short of these highs.
Scotland, Northern Ireland and Wales - along with northern and western England - will be significantly cooler with afternoon highs expected to reach 16-24C (61-75F).
You can check the forecast temperatures for your area on the BBC Weather website and app.
Sunday will bring a return to hot weather, especially in the south and east
Whether or not we reach official heatwave criteria, temperatures will start to climb again on Sunday.
Sunny spells are expected in most areas and temperatures are expected to peak at 27-31C (81-88F) in parts of the Midlands, Wales and southern England.
Some areas of northern England will reach 23 or 24C (73-75F) while much of Scotland and Northern Ireland will be at 20-22C (68-72F), away from the far north.
High UV and pollen levels are anticipated in many places.
Extreme heat on Monday and Tuesday
The Met Office amber warning for extreme heat, external will come into force across much of southern and eastern England and parts of south Wales from 01:00 BST on Monday.
This remains valid until 23:59 on Tuesday, with temperatures set to peak at 34 or 35C (93-95F) in south-east England.
It is very likely that a heatwave will be officially declared in some areas.
This is only the sixth amber extreme heat warning the Met Office has issued since they were introduced in 2021
There will be tropical nights in some locations where overnight temperatures are expected to remain above 20C (68F).
This combination of hot days and very warm nights is likely to bring a range of impacts to people and infrastructure.
There is a risk to health, especially for elderly and vulnerable people.
The UK Health Health Security Agency has already issued separate amber and yellow heat health alerts across large swathes of England.
However, even in younger and healthier people there is a risk of health conditions such as sunstroke and heat exhaustion.
Delays and disruption to travel networks and power supplies are possible.
There is also the potential for scattered intense thunderstorms, especially for parts of England and Wales, which could pose an additional hazard.
When will the heatwave end?
Some forecasts suggest temperatures could hover around 30C for at least seven consecutive days in places, perhaps rising back into the mid-30s at times although there is some uncertainty.
This current hot spell follows a record breaking May where the UK recorded its warmest May day.
The Met Office says heatwaves are becoming more frequent and more intense globally due to human-induced climate change.
Scientists project that hot spells will become more common in our changing climate, particularly in the south-east of the UK, saying it is "virtually certain that human influence has increased the occurrence and severity of extreme heat events".
The developing El Niño is not responsible for this current hot spell, with the peak effects expected later this year and into 2027.