Video report by ITV News Correspondent Juliet Bremner
The UK enjoyed the hottest day of the year so far on Thursday, triggering a major incident to be declared after "half a million" visitors headed to the south coast of England.
Bournemouth, Christchurch and Poole council said services were "completely overstretched" as huge numbers of visitors defied advice to stay away - prompting authorities to declare the major incident.
Bournemouth East MP Tobias Ellwood said Dorset had seen "half a million visitors" to the area causing traffic jams and full beaches.
Dorset Police urged people to stay away adding: "We are still in a public health crisis and such a significant volume of people heading to one area places a further strain on emergency services resources."
Scenes of busy beauty spots up and down the country prompted health secretary Matt Hancock to warn the government has powers to close public areas like beaches if social distancing rules are not being observed.
Asked if he would consider shutting some beaches in extreme cases after scenes in Bournemouth, Mr Hancock told TalkRadio: "Well, we do have that power."
But added: "I am reluctant to use it because people have had a pretty tough lockdown. Everybody should be able to enjoy the sunshine.
"The key is to do it with respect. Stay with your households. Stay a good distance from other households.
"We do have those powers – and if we see a spike in the number of cases, then we will take action."
His warning echoed that of the chief medical officer for England who said: "If we do not follow social distancing guidance then cases will rise again."
Professor Chris Whitty added: "Covid-19 has gone down due to the efforts of everyone but is still in general circulation. Naturally people will want to enjoy the sun but we need to do so in a way that is safe for all."
ITV News Science Editor Tom Clarke said Professor Chris Whitty may have warned the public to follow social distancing guidelines due to a "flatlining" in the decrease of coronavirus cases in England
The Met Office officially declared Thursday as the hottest day of the year so far after the temperature reached 33.3°C at Heathrow Airport - beating Wednesday’s high of 32.6°C at the same location.
The soaring temperatures saw "thousands" of beachgoers descend on the south coast.
Bournemouth, Christchurch and Poole council leader Vikki Slade said the council had "no choice but to declare a major incident" as beachgoers flocked to the area.
"We are absolutely appalled at the scenes witnessed on our beaches, particularly at Bournemouth and Sandbanks, in the last 24-48 hours," she 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."
The council said 558 parking enforcement fines had been issued - the highest on record.
Dorset Police Assistant Chief Constable Sam de Reya said the force was increasing patrols in the area.
"These are unprecedented times and we are urging people to stay away from the area of Bournemouth beach and other Dorset beaches.
"We are also deploying additional resources to provide increased patrols in the vicinity to help tackle any issues of anti-social behaviour and other offences being committed.
"We are also reliant on people taking personal responsibility and strongly advise members of the public to think twice before heading to the area. "
In Sussex, police were called to a "serious incident" on Worthing seafront on Thursday afternoon.
A woman, believed to be in her 50s, died at the scene following what police described as a "medical episode" - temperatures had reached 28°C in Worthing by Thursday afternoon.
Authorities have warned the public not to be tempted to cool off in open water during the hot weather.
It follows 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.
While Thames Valley Police said on Wednesday evening they had found a body following a search for a missing man at Lulle Brooke in Cookham, Berkshire.
The man, who was in his 30s, was understood to have got into difficulty in the water on Tuesday evening.
The Met Office's amber level three heat alert remained in place forYorkshire and the east and south of England as well as the West and East Midlands on Thursday.
Health authorities encouraged those most vulnerable - many of whom have been shielding during lockdown - to protect themselves amid the "exceptionally hot weather forecast".
While people have been advised to keep cool and stay hydrated where possible.
Visitors have flocked to the UK's beaches:
The good weather is set to change, however, with thunderstorms forming across the west of the UK on Thursday afternoon.
Stormy weather is set to form in the south west and south east of England, Wales, western Scotland and Northern Ireland from 4pm.
The Met Office has issued a yellow weather alert for those areas until 9am on Friday, warning of disruption due to flooding, lightning strikes, hail and wind.
Between 30mm to 40mm of rain could fall in less than two hours.
The storms are expected to continue from midday on Friday through to 6am on Saturday, with another yellow warning in place for the whole of the UK.
Areas hit by the storms could experience "torrential downpours" with between 30mm to 50mm of rain falling in an hour.