Drivers are being warned to take extra care on the roads as thunderstorms are expected to bring torrential rain to parts of the UK overnight.
The UK experienced its hottest day of the year so far on Thursday, with the mercury rising to 33.4C (92.12F) at Heathrow Airport in west London.
The heatwave saw thousands of people flock to the south coast, resulting in a major incident being declared and Government experts warning people against gathering in large groups.
Storms have already started to develop in the north-west of Scotland, Wales and south-west England, and are expected to continue overnight.
A yellow weather warning is in place for Northern Ireland, Wales and the west of Scotland and England, with the storms set to move eastwards into Friday.
Met Office meteorologist Craig Snell told the PA news agency: “If you live in those areas, don’t be surprised if you get woken up by a few rumbles of thunder tonight.
“If anyone is driving overnight tonight, they should expect there to be a lot of surface water spray on the roads locally.”
Between 30 to 40mm (1.6 inches) of rain could fall in less than two hours in storm-hit areas, bringing a risk of flooding and disruption to some places, the Met Office said.
Mr Snell said the evening will also be “quite uncomfortable” for those trying to sleep as temperatures hover between 19-20C (68F) overnight on Thursday.
The storms are expected to continue from midday on Friday through to Saturday morning, with another yellow warning in place for the whole of the UK.
Scotland and Wales also experienced their hottest days of the year so far, with the former recording 30.8C (87.44F) in Prestwick, and the latter seeing temperatures rise to 31.4C (88.52F) in Trawsgoed, near Aberystwyth.
Bournemouth, Christchurch and Poole council said it was left with no choice but to declare a major incident after services were “completely overstretched” as huge numbers of visitors defied advice to stay away during the hot weather in recent days.
Condemning the “irresponsible” behaviour of crowds who gridlocked roads, dumped rubbish, abused refuse collectors and parked illegally, the council said extra police patrols have been brought in.
Chief medical officer for England Professor Chris Whitty urged people to follow social distancing rules in the hot weather or risk causing a spike in coronavirus.
He wrote on Twitter: “Covid-19 has gone down due to the efforts of everyone but is still in general circulation.
“If we do not follow social distancing guidance then cases will rise again.
“Naturally people will want to enjoy the sun but we need to do so in a way that is safe for all.”
Health Secretary Matt Hancock also tweeted: “We all want to be outside and enjoying the glorious sunshine but if you are heading out please do it safely and stick to social distancing rules.”
Bournemouth East MP Tobias Ellwood echoed calls from Dorset Police and the council for people to stay away and said he had asked the police minister to send additional officers if the force requested it.
Dorset Police Federation said officers had been dealing with “drunkenness, fights and having to locate missing children as pandemonium hit Bournemouth beach”.
Council leader Vikki Slade said: “The irresponsible behaviour and actions of so many people is just shocking and our services are stretched to the absolute hilt trying to keep everyone safe. We have had no choice now but to declare a major incident and initiate an emergency response.
There have also been warnings not to be tempted to cool off in open water, following the deaths of two people in recent days in Berkshire and Scotland.
A 10-year-old boy died after getting into difficulty in Loch Lubnaig, near Callander in the Highlands, on Wednesday evening, Police Scotland said.
Meanwhile, a body was found following a search for a missing man at Lulle Brook in Cookham, Berkshire.
A cousin told the PA news agency that Syrian refugee Eyad Al Ryabi had gone into the water on Tuesday evening to try to help his friend.
The friend survived and was taken to hospital in a serious condition, Thames Valley Police said.
The National Fire Chiefs Council (NFCC) tweeted: “Although the weather is hot, the water is not. Cold water shock can take your breath away please do not be tempted to cool off in open water and #BeWaterAware.”
An amber level three heat-health alert, issued by the Met Office, was extended on Thursday to take in Yorkshire and the east and south of England as well as the West and East Midlands.
Public Health England (PHE) said older people, those with underlying health conditions, and very young children were all more at risk from the higher temperatures.
Meanwhile, the AA has urged drivers to check their tyre pressure and coolant levels before leaving home following an increase in heat-related breakdowns.
AA president Edmund King said: “With the combination of the severe heat, cars that have been in lockdown and millions of vehicles that have delayed their MoTs, the AA has seen more breakdowns.”