The UK has recorded its all-time highest May temperature as part of London reached 34.8C on Monday.
That provisional figure, recorded in Kew Gardens, also surpassed the hottest bank holiday Monday on record - 33.3C on the August bank holiday in 2019.
Wales has also experienced its hottest May day, reaching 32.2C at Hawarden Airport in Flintshire.
"This heat would be exceptional in the UK even in mid-summer, let alone in May", the Met Office said.
The mercury is expected to rise further in the South and South East of England, with forecasters warning of possible 35C heat on Monday and Tuesday.
The UK recorded its warmest May night on Sunday, with highs of 19.4C in London.
This week's weather means more than half of the monthly record highs - seven out of 12 - have been set since 2003, according to the Met Office.
These exceptionally high temperatures have been caused by "the influence of warmth building under an area of high pressure near the UK", Met Office chief operational meteorologist Dan Suri said.
The average temperature for the end of May is 14-20C.
On Friday, the UK Health Security Agency issued its first amber-heat health alert of 2026, covering much of England, including the West Midlands, East Midlands, East of England, South East and London.
The North East, North West, South West, Yorkshire and Humber regions are under a yellow alert. Both alerts will remain in place until 17:00 BST Wednesday.
Scotland and Northern Ireland recorded their highest temperatures for 2026 so far on Monday - 25.5C in Charterhall and 24.6C in Hillsborough respectively.
Around 500 properties in Sussex and Kent were left without water, or had intermittent supply due to "increased demand", on Monday afternoon.
Apologising for the disruption, South East Water said it was working "as quickly and safely as possible" to resolve the issues.
Across the UK a number of bank holiday events were cancelled because of the extreme heat.
In Surrey, Leatherhead Lions Club said its annual bank holiday donkey derby would no longer include the donkeys or a dog show "for the welfare and comfort of the animals".
Despite these cancellations, thousands of people gathered in the baking heat to watch competitors from all over the world battle it out at the Gloucestershire cheese-rolling race.
German YouTuber Tom Kopke, known online as Tooleko, beat local competitor and Guinness World Record holder Chris Anderson in the first race of the annual event on Cooper's Hill in Brockworth.
As the hot weather continues, England could see some thunderstorms on Tuesday evening, forecasters have said.
Temperatures are expected to hit 30C on Wednesday and Thursday in England and Wales before dipping into the high 20s on Friday.
Heatwave conditions were met in eight parts of England on Sunday - Heathrow in Greater London, Benson in Oxfordshire, Brooms Barn in Suffolk, High Beech in Essex, Kew Gardens and Northolt in London, Santon Downham in Suffolk and Writtle in Essex.
The AA has warned drivers "not to drop their guard" when setting out on long journeys this week, with vehicle interiors possibly reaching 60C when in the sun.
"Drivers should never leave children, vulnerable passengers or pets inside a parked vehicle, even for a short time," the breakdown service said.
Age UK recommended staying inside between 11:00 and 15:00, and taking regular cold baths and showers.
Climate change is believed to have played a 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.
It is not just the UK experiencing this historic heatwave, with several European countries seeing May heat records broken over the last few days.
In Spain, 38C was recorded on Sunday. Across France and Germany, with temperatures climbing into the mid-30s, hundreds of localities saw records for May being broken.
Temperatures across France, Germany, Switzerland, Spain and northern Italy have been 10 to 15C above average for late May - and with the unprecedented heatwave expected to last for most of the week, it is likely more local records will be broken.
