This would make it the hottest day of the year so far, beating the previous record of 19.7C set earlier this month.
The best of the warm weather is set for the south, but as far north as Glasgow could see temperatures in the mid-teens still.
Forecasts would place the UK hotter than both Ibiza and Corfu – the latter of which is set for around 16C on Thursday, but warmer temperatures early next week.
There’ll be plenty of sunshine this week and daytime temperatures are on the rise, becoming several degrees above average.
On Wednesday, 17C is possible in Wales, the Midlands and southern England.
On Thursday, temperatures will widely reach 16-19C, perhaps climbing to 20C in England and Wales.
That would be the highest temperature of the year so far, and on the same day as the spring equinox – the start of the astronomical spring.
It will still be warm for most on Friday, but the highest temperatures will be further north and perhaps aided by the Foehn effect – warming and drying of air on the lee side of a cross-mountain wind.
Northern England and Scotland looks most favoured for 20C on Friday.
Temperatures might not be quite as high over the weekend, but should remain above average, despite the weather turning more unsettled.
There’ll be a widespread frost over the next few nights, with known cold spots falling well below freezing and -9C is possible in northern Scotland on Tuesday night.
Weather over the next few days
Widespread clear skies over Tuesday will lead to a cold night into Wednesday, with temperatures dipping below freezing for the vast majority.
In parts of northern England and Scotland, temperatures could be as low as -9C.
Image: People enjoy the sunshine as they punt along the River Cam in Cambridge.
Pic: PA
Image: People walk past beach huts in sunny weather in Bridlington, Yorkshire.
Pic: PA
But the increasingly strong March sunshine will warm the day up into the high teens.