High temperatures and sunshine are predicted for the first week back at school for all pupils across the UK.
Despite the official meteorological end of summer, warm weather is expected to last for several days, with the mercury hovering around the mid-20s in many parts of the country.
But the Met Office said it is still “touch and go” whether some areas will tip into official heatwaves.
Temperatures are predicted to reach highs of 28C (82.4F) in London on Monday and rise to 29C (84.2F) on Tuesday.
Northern areas are also expected to see an increase, with Liverpool seeing highs of 23C (73.4F) on Monday and 27C (80.6F) on Tuesday.
Both Northern Ireland and Scotland are set to share the pleasant conditions, with highs of 23C (73.4F) and 22C (71.6F) predicted in Belfast and Aberdeen respectively on Monday.
The highest temperature recorded on Sunday was 27.1C (80.8F) at Wiggonholt, west Sussex, according to provisional data from the Met Office.
The warm weather follows the official end of summer, from a meteorological point of view, which is considered to be August 31.
It comes as children across the country return to classrooms after the summer break. Many schools in England and Wales started again last week and pupils in Scotland and Northern Ireland are already back.
The Met Office said the warm spell is expected to last until Wednesday, and predictions show some areas are on the “borderline” of experiencing an official heatwave.
A location meets the UK heatwave threshold when it records a period of at least three consecutive days with daily maximum temperature levels which vary across the country.
These include 25C (77F) for central England and Wales – where it has been forecast the threshold could be exceeded – and 28C (82.4F) for London and the South East.
Met Office spokesman Stephen Dixon said: “It’s not uncommon for the UK to get warm spells heading into early September.
“There’s a chance of some areas reaching heatwave criteria, but that is fairly borderline and the breakdown that is happening on Wednesday will subdue the temperatures as well.
“So it will be touch and go for some places whether a heatwave is officially in the forecast.”
He added: “Temperatures (on Monday) are going to be widely in the mid-20s, especially in the south, where there’s going to be spells of good sunshine.
“Heading into Tuesday, that warmth spreads further north across the country, bringing a fine and dry day for most with that warm weather extending across the UK.”
Mr Dixon said that, although good conditions will last until Wednesday, “rain and instability” will follow, leading to possible thunderstorms in south-west England.
The mercury is expected to exceed the average for September, which is 18C (64.4F) in the UK.