The UK is expected to bake this week, with temperatures in some places predicted to top Rome, Venice and Madrid.
But Britons should make the most of the sunny weather as it could become unsettled as early as next week, the Met Office said.
Tuesday was the warmest day of the year so far, with 20C (68F) recorded in Gravesend, Kent, but the mercury will keep rising with an outside chance of 26C (79F) in the South East on Thursday or Friday.
The mini-heatwave means temperatures could rise more than 10C (50F) above the April average, just weeks after snow covered swathes of the UK.
Meteorologist Sophie Yeomans said: “In the next few days we could see the warmest day of the year come day-on-day as temperatures build.
“Warm air is coming up from the western Mediterranean, up through Spain and reaching south-eastern parts of the UK.
“It was cloudy and rainy today in some places, but by tomorrow most places will brighten up.
“The peak temperatures are going to be Thursday or Friday when we could see highs of 25C (77F) with the outside chance of 26C (79F) somewhere.”
Scotland and Northern Ireland should shed most of their cloudy and rainy spells by Wednesday and start to see fine and sunny weather as the week progresses, she added.
Average maximum temperatures for April across the UK are around 11.5C (53F) and around 13C (55F) for southern England, according to the Met Office.
Ms Yeomans added: “I think by the end of the weekend we could see a few showers coming in so it’s probably best to make the most of it this week.
“It could become unsettled by next week, perhaps with outbreaks of rain, especially in the North West.”