Experts say weather warnings could be issued from Tuesday as "dreary" showers and thunderstorms replace two weeks of Mediterranean-style sunshine.
Heavy rainfall could see up to 1.6ins falling in a single day, leading to floods and travel chaos.
Met Office forecaster Emma Salter said: "Rain will set in on Tuesday across parts of England and Scotland and there will probably be a few warnings issued as showers could be heavy and persistent.
Temperatures have soared over the past few weeks
Experts are warning there will be heavy rain
People are being told to brace themselves for bad weather
"It is looking unsettled and changeable now this week and probably through next week as low pressure influences our weather.
"It is a return to the typical, and at times pretty dreary, British summer weather."
The forecast will disappoint those who enjoyed basking in Mediterranean-style sunshine over the last two weeks.
Manchester will see significant rainfall overnight and temperatures could fall as low as 12C on Wednesday.
There will be heavy rain in Newcastle tomorrow morning which will continue throughout the day.
Liverpool could experience its worst weather this week on Wednesday, when rain is expected to fall all day.
Holidaymakers hoping for a sunny getaway in Cornwall and Devon will also be disappointed as the mercury drops as low as 15C during the day along the south coast.
Travel warnings are likely to be issued and those heading to music festivals across the UK at the weekend could find themselves drenched.
People have enjoyed the hottest spell of the year so far
It's been so hot boys have turned up for school in skirts
James Madden, forecaster for Exacta Weather, added: "There will be some passing and potentially vigorous thunderstorms, these may be accompanied by some hefty hail showers and some flash-flooding from high rainfall.
"As we head into July a more mixed and cooler pattern of weather is likely for most."
Last week brought the longest June heatwave since the scorcher of 1976, peaking as 34.5C was recorded at Heathrow Airport.
Forecasters say temperatures in July are likely to remain around the 20C mark - but further heat waves are not out of the question, thanks to hot weather blowing in from Spain.
Dr Todd Crawford, chief meteorologist at the Weather Company, said: "Three or maybe four more heat bursts from southern Europe are expected in the UK this summer.
"We do also expect variability this summer in the UK, with extreme heat and sun but also cooler and wetter spells."