The Met Office has warned of an "unusually" hot heatwave heading for the UK this week.
Temperatures are likely to soar to 33C when a "Spanish plume" hits the UK on Friday, forecasters said.
The previous hottest day of the year took place on May 17 when the mercury reached 27.5C at Heathrow.
The predicted highs are well above average for this time of the year, making Britain hotter than Hawaii.
Met Office Deputy Chief Meteorologist Dan Rudman said: “Temperatures will rise through the week, becoming well above-average by Friday when many parts of the southern half of the UK are likely to reach 30C or even 33C in isolated spots.
“This is the first spell of hot weather this year and it is still unusual for temperature to exceed these values in June.
"Many areas will also see some warm nights with temperatures expected to be in the mid to high teens overnight.”
The UK has not experienced temperatures in the mid 30Cs in June since 1976.
The unseasonably hot weather is expected to start on Wednesday, when the mercury will creep up to 27C in places.
In its prediction for Wednesday, the Met Office said: "Becoming hot in the south with some strong sunshine. Cooler and cloudier in Scotland and Northern Ireland."
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The forecaster said Thursday is expected to be another "hot and sunny day" for much of England and Wales with highs of 27C, although temperatures will be cooler in the north-west.
On Friday, the Met Office said: "Hot air from the continent will bring temperatures of around 30C in the south and some strong sunshine."
Temperatures could hit 25C in Manchester, 21C in Edinburgh, 22C in Belfast and 23C in Cardiff.
Meanwhile pollen counts will be "high to very high" for much of England and Wales throughout the week, while low to moderate counts are expected across Scotland and Northern Ireland.
The weather service said: "It is peak grass pollen season so hay fever sufferers may need to take precautions."
Heat has been pushed from the north African desert into Europe, bringing sweltering conditions to Spain and France.
The Spanish meteorological agency last week issued warnings of “extreme” temperatures of 40C plus in some places.
The Met Office had warned that ex-tropical Storm Alex would cause strong winds and showers, especially for western Scotland and Northern Ireland, on Friday and Saturday.
The weather for the rest of today is expected to be dry with some bright and sunny spells in England and Wales, while Northern Ireland and Scotland may see some showers.
UK 5-day forecast
Monday
Fine for England and Wales with sunny spells. Scotland and Northern Ireland cloudier with some light rain - most frequent in northwest Scotland but heavier shower possible in northeast Scotland too. Feeling fairly warm in sunnier southeast. Some areas of cloud but largely clear and locally chilly in south. Cloudy in the northwest with some patchy rain.
Tuesday
Much of Scotland and Northern Ireland cloudy with some rain at times, especially across the northwest. Elsewhere, mostly dry with variable cloud and sunny intervals. Feeling warm in the sunshine.
Wednesday to Friday
Continuing cloud, rain and breezy conditions across the northwest. Elsewhere, mostly dry with sunny spells and becoming increasingly warm. Potentially hot by Friday in the south with some isolated showers.
