The Met Office said rain, and possibly thunderstorms, would come to much of Britain after the latest heatwave caused temperatures to soar as high as 38C last week.
Following the recent spell of hot weather, Wessex Water became the latest water firm to announce a hosepipe ban on Monday, with the measure coming into effect from midnight on Tuesday.
The restrictions, impacting Bath, Somerset, Dorset and Wiltshire, mark the first time the supplier has imposed a ban in 50 years, and affects all 1.4 million of its customers.
The cooler weather comes as a jet stream of air in the country's north that allowed high pressure to dominate, resulting in last week's excessively hot conditions, travels south, the forecaster said.
This shift will cause low pressure to dominate and drift southwards, resulting in an unsettled outlook in the middle of the week and temperatures to fall as the wind switches to a northerly direction.
The temperature will slip into single digits in some locations, including in the Brecon Beacons in Wales, where lows of 6C are forecasted for the early hours of Saturday, and South Newington in the Cotswolds, where a low of 8C is predicted.
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Met Office meteorologist Tom Morgan said: "I can't promise rain for all of us through the week ahead, but in the south at least, it's looking like the most changeable week we've seen here - well at least since the end of June."
Mr Morgan said Britons will see some "quite menacing skies" with "dark clouds one moment and some sunshine the next".
Rain is expected in Northern Ireland and western Scotland on Monday, with some more showers in England's South East early in the day.
Some parts of the country, especially in the South West, will see temperatures well into the 20s, including predicted highs of 28C and 26C in London and Southampton respectively.
More rain is forecasted for Tuesday, with the UK's north and central regions warned to expect a wet start to the day, with the showers moving "erratically" southwards throughout the day.
Wednesday will be dry in the south of the UK, but there will be a mix of sunshine and rain in the rest of the country.
Showers will become more widespread and heavier from Thursday, with a chance of thunderstorms.
The UK faced heatwave conditions last week, with the temperature hitting 38.1C in Kew Gardens, west London, last Thursday - making it the hottest day of the year so far.
Wildfires have ravaged parts of the UK, including the West Midlands, where a blaze in Stourbridge destroyed 19 homes.
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West Midlands Fire Service said they had been able to lift cordons as households who had been evacuated started returning to their homes over the weekend.
South Wales Fire and Rescue Service also responded to "significant wildfire activity" on Sunday, with crews attending 14 blazes.
About 200 military personnel and crews from London, Greater Manchester, Cumbria, and Bedfordshire were deployed to the region to assist firefighting efforts.
Millions of people in England and Wales received an alert on their mobile devices warning them of a "very high risk of wildfires nationally" on Friday.
The alert came after Prime Minister Andy Burnham described Britain as a "tinderbox" and put in place a temporary ban on disposable barbecues.
Fire services across the country have reported high levels of demand, including South Wales Fire and Rescue Service which said on Saturday it had received 736 calls compared with an average of about 300.
The hot weather has seen hose pipe bans imposed across the country, including from Thames Water, Southern Water and Anglian Water.
Two cars were in a head-on collision on the M9 between junctions 3 and 4 near Athy at around 3am on Sunday.
One of the vehicles was travelling in the wrong direction - southbound on the northbound carriageway of the motorway, according to police.
Gardai were told not to engage with the vehicle, which was allegedly stolen and had previously failed to stop for officers, a police representative group said.
The bodies of the five teenage males were removed from the scene of the crash and taken to Naas General Hospital for post-mortem examinations, the gardai said.
Four people remain in hospital with serious injuries, police said.
Garda Representative Association (GRA) vice-president John Joe O'Connell said the crash involved a BMW that had failed to stop for gardai and an instruction had been given not to engage with the vehicle.
"It would be standard practice, and the command and control centre would be dictating what happens in a live environment - but the members in the cars have to make a split-second decision," he told RTE's Morning Ireland radio programme.
Mr O'Connell said the GRA did not "know what the exact answer is" to address the problem of gangs "trying to outdo each other" in social media videos depicting interactions with Gardaí, pursuits and wrong-way driving.
"Our members are not going to follow down that route - we're damned if you do and damned if you don't, that way," he said.
Mr O'Connell said the people in the other car who were injured were a family travelling to Dublin Airport for a holiday.
Garda Commissioner Justin Kelly said: "These family members were travelling innocently when they were struck by a vehicle being driven the wrong way down the motorway.
"This horrific incident, following the recent collision on the M50, shows how irresponsible and dangerous driving on the wrong side of the motorway is.
"That, of course, should be self-evident, but what makes this reckless behaviour even more appalling is that, in some cases, it is being done for social media likes regardless of the potentially devastating impact it might have on innocent people and themselves."
He added: "As this particular incident has been referred to Fiosrú, the Police Ombudsman, I am constrained in what I can say, but I know that Gardaí do what they can to stop such vehicles while also taking into consideration all other road users."
The four people taken to hospital include the female driver and front passenger of the second car. They are both aged in their 30s and remain in a critical condition.
A female rear seat passenger, aged in her 20s, is in a serious condition.
A boy, who was a passenger in the car, is also in a serious condition.
The M9 from junction 3 to junction 4 northbound and southbound remains closed, and a forensic investigation is set to take place.
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A referral has since been made to Fiosru, the Office of the Police Ombudsman.
It said in a statement: "The referral was made following a fatal road traffic incident on the M9.
"One of Fiosru's on call team of investigators attended the scene and its independent investigation is ongoing.
"In line with its statutory remit, Fiosru's investigation relates to incidents involving contact with gardai (Irish police) where a member of the public has died or been seriously injured. A referral does not necessarily mean a garda or gardai have been accused of wrongdoing."
Officers have asked for any witnesses to come forward.
They are also appealing to those who were travelling on the M9 northbound or southbound between 2am and 3am on Sunday morning and who may have dashcam footage of the incident.
The US president had asked the court to reconsider its decision in June that rejected his initial appeal.
It is unusual - although not unheard of - for the court to grant such requests.
Trump paid the $5m shortly after the court's decision.
The case arose from a federal lawsuit Ms Carroll filed in New York City alleging that Mr Trump sexually assaulted her in the dressing room of a department store in 1996.
The defamation claims relate to statements he made about her after his first term in office, in which he described her claims as a "con job" and "hoax".
Trump has denied any wrongdoing and has said he does not know Carroll. His lawyers contend that the trial was unfair.
His lawyers in the separate appeal for the $83.3m jury verdict for defamation argue that presidential immunity shields him from Ms Carroll's claims and that lower courts wrongly decided he had forfeited that defence.
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The 29-year-old man, from Kansas, was on the active volcano in Sicily, Italy, during a thunderstorm, local media reports.
He was struck on Sunday evening in an exposed area above the treeline.
The victim was flown by helicopter to hospital and later pronounced dead by medical personnel. His identity has not yet been confirmed.
More people have been killed on Etna by lightning strikes in recent decades than volcanic activity - with the last eruption-related deaths recorded in 1987.
Vast areas of the volcano's slope are off limits to hikers due to its recent activity - which led to the temporary closure of the airport in nearby Catania.
Hundreds of holidaymakers were left stranded during the busiest travel week of the year as ash spewed from Etna.
The latest activity at the largest active volcano in Europe has been unusual for its persistence and sustained ash emissions.
Northerly winds have spread ash as far as Malta and northern Africa.
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Boris Behncke, a researcher at the National Institute of Geophysics and Volcanology's Etna Observatory, said: "Volcanic ash can be a great risk for aircraft.
"It can be sucked into the motors, and force them to stall.
"This is why everyone is so very cautious with aircraft when there is a volcanic ash emission."
The airport's closure meant planes were rerouted to other airports on the island, which increased taxi fares and car rental prices.
Emergency services were called to Hackney Marshes after the boy was seen entering the River Lea at about 6pm on Saturday, the Metropolitan Police said.
His body was recovered by police divers just after midnight on Sunday, and he was pronounced dead at the scene.
The boy's next of kin have been notified, police said.
Mayor of Hackney Zoe Garbett said she was "heartbroken" to learn of the boy's death.
"All of our thoughts are with the child's family and friends at this incredibly difficult time," she said.
"This is a devastating loss for our community, and I know many will be feeling a deep sense of shock and sadness.
"The boy's family will need time to grieve this unimaginable loss, so please respect their privacy."
The detective leading the investigation, Detective Inspector Azeem Pinnu, said: "This is a deeply tragic incident, and our thoughts are with the boy's family, friends and the wider community.
"We also make a plea to the public to avoid speculation and refrain from sharing distressing material online out of respect for the family and to preserve the investigation.
"This incident is now being investigated on behalf of the coroner, and inquiries remain ongoing to establish the full circumstances of what happened."
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Much of the UK experienced warm weather over the weekend during the final days of a heatwave.
The temperature hit 38.1C in Kew Gardens, west London, last Thursday - making it the hottest day of the year so far.
The boy's death follows a spate of drownings in rivers, lakes, and other bodies of water during the recent heatwaves.
During the May heatwave at least 15 people died swimming in open water, many of them teenagers and young adults.