The mercury is forecast to drop from 29.1C last Thursday – the hottest April day since 1949 – to averages of just 11C by Wednesday.
And higher areas of Scotland and Northern England could see lows of -5C as well as the return of snow and ice when our weather "rewinds to winter".
The stark shift comes after the hottest London Marathon in the race's 38-year history on Sunday as brave runners endured exhausting 23C temperatures and blistering sunshine.
Medics treated dozens of people suffering symptoms of heat exhaustion as forecasters warned that warmth coming from the tarmac would make it feel even hotter.
The 40,000 taking part were urged by organisers to rethink fancy dress and stay hydrated as well as consider slowing their pace to stave off overheating.
Other parts of the southeast enjoyed temperatures in the mid-twenties including Gravesend in Kent which hit 24.5C.
But Met Office forecaster Becky Mitchell said today: "After last week's high temperatures it's going to be a lot cooler this week.
"Temperatures will return to the April average of 52F (11C)."
Already parts of the UK suffered unsettled conditions as hot humid air brought thunderstorms and heavy rain.
Dramatic photos showed electric storms breaking over Bournemouth, Dorset, The Isle of Wight, and Hampshire as well while rain showers affected areas across the country.
Cold air is now set to push in from Iceland and arctic weather fronts will dominate conditions throughout this week.
Met Office forecaster Greg Dewhurst told the Mail Online: "After a warm Sunday for the Marathon, there's a big change ahead, with much cooler temperatures and rain or showers."
And Weather Outlook forecaster Brian Gaze added: "We'll rewind towards wintry weather. The weather will crash back down to earth.
"A giant drop in temperatures is ahead after the heatwave. Highs will be 15C or so lower than leading into the weekend.
"Scotland could see snow on mountains, with -5C in glens, and 0C in northern England from midweek, and frost."
Craig Snell from the Met Office said of Monday's weather specifically: "A good chunk of England and Wales will be dry.
"Further West it will be cloudier and rain will move in from the North, particularly affecting Scotland and Northern Ireland.
"The rain will then move down the country throughout the afternoon, especially hitting the North West, Wales and the South West.
"It will feel much colder this week. At the absolute best it will be 17C in the South East — a good 10 degrees colder on this weekend.
"It definitely wont be feeling as warm at all. In the North there will be minimum temperatures of 5C, while in the South it will be around 8C."
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