The UK has recorded the hottest day of the year so far as temperatures climbed to 27.5C.
Forecasters said the mercury rose to its highest level at Heathrow Airport in west London.
The figure means that the UK was warmer than Santorini - which only saw 26C on Tuesday.
But clouds and heavy rain in the west will spread into central areas of the UK into Tuesday evening, the Met Office anticipates.
The highest temperature recorded previously this year was 23.6C in Faversham, Kent, on 6 May.
Forecasters said Tuesday's warm weather was the result of an air mass from the south bringing up mild air, particularly impacting areas in the South East.
However, temperatures will slowly decline as the week goes on and the air mass moves, bringing in fresher air and temperatures closer to the average for this time of year.
Met Office forecaster Oli Claydon said: “We’ve got an air mass moving up from the south which is helping to bring some warm air with it and why we’re seeing some of the highest temperatures of the year.
“It’s the same reason why we’ve been seeing the thunderstorms over the past few days as well. That will change as we go through the week, with fronts moving in and bringing ever so slightly fresher air.
“One thing to note is that the particular high of 26C is quite confined to the South East.
“For example, in Northern Ireland, temperatures will be stuck in the mid-teens. Belfast today will see 15C as its high today, so there is quite a contrast across the UK.”
The Met Office’s Richard Miles said: “Today you might see up to 23C in the south-east so it won’t be so outlierish tomorrow, maybe up to 25C and possibly 26C where the sunshine really hangs around in the south-east.
“Obviously the warmest temperatures of the year so far, it is largely confined to the south-east of England; other areas will be generally fine but not quite as warm.
“There is a risk, certainly today, of quite a bit of rain around.
“Tomorrow, drier, although there will be some convective showers.”
He added: “The main reason for the warmth will be the higher pressure of the continent which is where the plume of warmer air will reach into the south of the UK, so it is basically a warm air flow from the continent to the south, but it is being pushed further eastwards then maybe it looked last week.”
Mr Miles said the balmy temperatures in the south-east will be met with the “westerly systems bringing the more unpredictable and showery weather”.
He added: “The story of the week will really be low-pressure systems in the Atlantic will be pushing up weather fronts in the western side of the country.
“Pushing and bringing up frontal rain and, in the south and the east of the country, where the warm air from the continent is causing some convection, that will produce showers in those areas.
“There are two different ways weather is delivered but everyone will get some, but tomorrow will be largely nice and the warmest day of the week.”
He predicted the weather will “generally be cooling” from Wednesday until the weekend, when temperatures will be “about the average for the time of year”.