Heatwave conditions have officially been reached at Heathrow Airport and in Charlwood, Surrey
Temperatures surpassed 28C for the third consecutive day, meeting the threshold for a heatwave - when is a heatwave declared?
More areas of south-east England and East Anglia will follow suit in the coming hours, our lead weather presenter reports
Amber heat health alerts are in place in parts of the Midlands and eastern England until at least Wednesday
It looks increasingly likely that a new all-time UK May temperature record will be set on Monday - 32.8C is currently the highest temperature ever recorded in May
Let us know what the weather is like where you live
Matt Taylor
Lead weather presenter
I can confirm that Heathrow Airport and Charlwood in Surrey are the first locations in the UK to reach heatwave criteria.
Temperatures reached over 28C in both locations for the third consecutive day.
More areas of south-east England and East Anglia will follow suit in the coming hours.
Here's a reminder of when a heatwave is declared.
Matt Taylor
Lead weather presenter
It is now almost certain that the existing UK national temperature record for May will fall over the next few days.
The current record of 32.8C (91F) was set on late spring bank holiday Monday in 1944 at Regent's Park, Horsham, and Tunbridge Wells.
Not only are we likely to exceed this in parts of south-east England on bank holiday Monday AND on Tuesday, but with forecast temperatures closer to 34 Celsius (93F) the record could be beaten by some considerable margin.
Wales could also set a new May record too in the next few days, which stands as the 30.6C (87F) recorded in Newport in 1944.
And it's not just daytime heat records at risk, but also nighttime ones. It's possible on Monday or Tuesday night that somewhere stays warmer than the May record of 18.9C (66F) set in Folkestone in 1947.
Here's some more detail on the amber and yellow heat alerts that remain in place across parts of the country.
The UK Health Security Agency (UKHSA) has issued the following alerts, in place until 17:00 BST on Wednesday:
Amber alerts
Yellow alerts
An amber alert means "significant impacts" are likely across health and social care services, while a yellow alert means "significant impacts" are possible.
As we explained earlier, the UKHSA says alerts are usually only issued between June and September, making the current ones "extraordinary".
Matt Taylor
Lead weather presenter
As temperatures soar this afternoon, some areas will officially be classed as being under heatwave conditions.
Heatwaves, as defined by the UK Met Office, are when certain temperature are reached for three consecutive days or more.
The actual temperature depends on where in the country you live. For many in the north and west of the UK the temperature threshold is 25 Celsius.
For much of central and southern England, as well as south-east Wales it's either 26 or 27 Celsius, but for London, parts of the Home Counties and Cambridgeshire it is 28 Celsius.
For large areas of south-east England and East Anglia we have already had two days above either 27 or 28 Celsius, and today is likely to be warmer still.
Image source, Met OfficePeople living in three villages in Kent are experiencing no water or low water pressure for the second day.
The affected areas are Charing, Challock and Molash near Ashford, where people first reported water supply problems on Saturday evening.
South East Water (SEW) said the issue was resolved overnight, but added that water supply problems resumed at about 09:25 BST on Sunday due to "issues with the pumping station".
The company has apologised to those who continue to be affected.
"We know water supply issues during hot weather are deeply frustrating and incredibly disruptive," a spokesperson says.
Matt Taylor
Lead weather presenter
In short ...yes!
Okay, we might have seen the temperature hit 30C in May in the past, but the intensity and longevity of the heat is something we haven't seen before.
30C days in May the UK are actually relatively rare; the last time being in 2012, with only around ten in total recorded over the last 126 years.
However, it is conceivable we could see five or six recorded across the UK this week alone!
That will not only be unusual at any point in the summer, but it will be an historical heatwave for late-spring.
Climate change is believed to play a large role in such hot spells as this.
According to the Met Office's State of the UK Climate report, the number of days in the UK with temperatures above 28C has more than doubled, and the number of days with temperatures above 30C has more than trebled in the most recent decade, compared with the 1961-1990 average.
While it's welcomed by some, hot weather comes with risks. The main ones are dehydration, overheating, heat exhaustion and heatstroke, the NHS says.
Certain groups are more at risk, including:
Tips for staying safe include drinking plenty of water, staying indoors, keeping to the shade (particularly between 11:00 and 15:00 BST), wearing sun cream, and avoiding activities that make you hotter.
As temperatures remain high on Sunday and a heatwave is expected to be officially declared in parts of the UK, let us know what it's like where you are.
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Amber heat health alerts, issued by the UK Health Security Agency (UKHSA) on Friday remain in place for the Midlands and much of south-east England.
Less-severe yellow warnings are in place for the remainder of England, with all alerts set to run to 17:00 BST on Wednesday.
These heat health alerts are issued by government agency UKHSA in partnership with the Met Office, and are only issued in England.
Yellow health alerts mean vulnerable groups will likely be affected, while amber alerts could affect the whole population and lead to "significant impacts" across health and social care services, says the agency.
The UKHSA lists on its website that alerts are generally only issued between June and September, but that “extraordinary alerts” may be issued if an episode of heat was to occur outside that window - as we are seeing now.
The threshold for hot weather becoming a heatwave depends on what the "normal" climate for an area is.
This is different across the UK, with the Met Office setting the temperature for a heatwave at 25C in Scotland, Northern Ireland and parts of northern England.
For many other areas of England, as well as south-east Wales, it is 26C or 27C. In London, most of the home counties and Cambridgeshire, it is 28C.
If these temperatures are exceeded for at least three days a heatwave is declared.
Jack Grey
Live reporter
Image source, Getty ImagesA heatwave is expected to be officially declared later today as temperatures soar across swathes of the UK.
On Saturday, a high of 30.5C was recorded in Frittenden, Kent, becoming the warmest day of the year so far, with temperatures set to remain high throughout Sunday.
Monday is forecast to be the hottest May day on record – with current forecasts showing the possibility of temperatures exceeding 33C in parts of south-east England.
For a heatwave to be declared, temperatures must exceed a threshold above the "normal" climate for three consecutive days.
Amber heat-health alerts remain in force for parts of the Midlands and eastern England until at least Wednesday
For Northern Ireland, Scotland, most of Wales and parts of England, this is 25C, while for parts of south Wales and much of England this ranges from between 26 and 28C.
We'll bring you the key developments right here on this page, stay with us.
