Temperatures have already reached 22C across parts of the South East on what is anticipated to become the hottest day of the year so far.
Parts of the country will be warmer than some Greek islands such as Santorini, Crete and Mykonos, and will even top Los Angeles in California by Tuesday afternoon.
The Met Office said this is 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 told the PA news agency: “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 highest temperature recorded so far this year was 23.6C in Faversham, Kent, on May 6.
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