Warm sunshine will see temperatures hit highs of 20C today - but rain will blight the start of the weekend for those in Northern Ireland, Scotland and parts of north England.
The Met Office forecast sunshine and pleasant temperatures for many in England and Wales during the day today.
The highest temperatures will be in Exeter where it should reach around 20C.
It won't be much cooler in London and Norwich - but Scotland could be as low as 10C.
A Met Office forecaster said: “It will be rather cloudy at first with outbreaks of rain affecting northern and some western parts.
“Sunny spells developing across England and Wales. Feeling very warm in the sunshine.
“Rain will become persistent and heavy at times, especially across western Scotland later.”
Fierce rain should be expected over Scotland, Cumbria and Northern Ireland while lighter showers will hit further south.
Then Saturday should see bumper high temperatures for all.
But the Met Office has warned that thunderstorms are expected to roll across the country on Sunday before cooler air moves in on Monday, dropping temperatures by about ten degrees in some places.
Met Office meteorologist Aidan McGivern said today will start “gloomy across much of northern and western Britain, damp across northern areas and with more wet weather pushing in through the rest of the day.
“Brighter and warmer weather developing further south across England and Wales, an improving picture here, but we’ve got to put up with one more day of persistent rain for Scotland and Northern Ireland.
“At least first thing there’s a lot of dry weather, even some sunshine in the far north.
“Later on it’ll be wet, with Northern Ireland seeing outbreaks of rain on and off first thing."
A spokeswoman for the Met Office said much of the country will enjoy balmy weather on Saturday - and Sunday could be even hotter.
Temperatures are then expected to jump and reach 27C in central regions and the south east.
The weather forecaster said: "If that does happen, it'll be the hottest day of the year.
"So far, April 19 holds the record, when temperatures reached 25.8C in Cornwall.
"Before then, things will be changeable. There'll be sunny and wet spells tomorrow.
According to the Met Office website, Barcelona and Ibiza will see highs of 24C (75F) on Sunday, while St Tropez is expected to reach 28C.
Ladbrokes are offering 10/11 odds that the hottest day of the year so far will be recorded this week.
A spokesman for the betting firm said: "Things will only get hotter over the next few days so it's brollies away and beach towels out with a scorching weekend on the way."
