Temperatures could reach as high as 25C in parts of the UK on Tuesday in a burst of late September warmth.
The warmest locations will be across the south-east of England, though temperatures will widely reach into the 20s Celsius across England and the high teens elsewhere.
This will be around 3 to 6 degrees above the average for the time of year.
Tuesday night will also be unusually warm with the temperature in some locations not falling much below 20C.
The boost in temperatures will come as a strong southerly airflow develops into Tuesday.
With unusually hot air across France and Spain - where temperatures have widely been in excess of 30C - the southerly wind will bring some of this warmth to the UK.
Temperatures across England will exceed 20C widely with the highest temperatures in south-east England where it will get into the mid-20s Celsius.
Average temperatures across England in late September are around 16 to 19C.
While there could be a lot of cloud with a few showers on Tuesday morning, sunny spells will develop over England and Wales.
It will also be warmer than average across Wales, Northern Ireland and Scotland with maximum temperatures on Tuesday of 17 to 22C.
However, after starting sunny here, cloud will increase during the day.
The southerly winds will strengthen with gusts up to 50mph expected in Northern Ireland and western Scotland.
This is linked to a large and deep area of low pressure moving north in the eastern Atlantic toward Iceland.
Nighttime temperatures may not fall below 20C
The warm southerly winds will continue overnight into Wednesday with minimum temperatures more typical of a summer's night than early autumn.
Widely, it will be around 15 to 19C.
In parts of south-east England, it is not out of the question that temperatures won't dip below 20C overnight.
Overnight temperatures in late September are typically around 7 to 11C, so this will make for an unusually warm night.
Another warmer-than-average month as September ends
September will mark another month where average temperatures will come in well above normal.
Up to 26 September, the Central England Temperature (CET) - the world's longest running temperature record, which tracks the average temperature across various locations in England - is 2.6 degrees Celsius above normal.
Every month this calendar year has seen the average mean CET above average. It is also 2.6C higher than normal for the year to date.
It would need to be much colder than normal during the rest of the year for 2026 to not be one the warmest on record for the UK.
The Met Office's latest three-month outlook, spanning October to December, suggests higher-than-normal chances of a mild autumn and start to winter.