It is the day that freshly suited and booted workers were due to emerge from home offices and return to the traditional commute.
But the heatwave means it is extremely unlikely that there will be a mass exodus of employees keen to get back to work on Monday as restrictions are finally lifted.
Instead, sun-drenched beaches and parks are expected to remain crowded as sweltering workers either call in sick or book the day off, while many families also keep their children off school to avoid self-isolation ahead of the summer holidays.
Sunday was the hottest day of the year so far in both England and Wales, the Met Office said, with temperatures reaching 31.6C at Heathrow and 30.2C in Cardiff.
The sunny weather is expected to continue until Thursday.
Forecaster Dan Stroud said: “We have a spate of very fine and settled weather throughout the UK which will persist into the week.
“From Thursday, there will be a degree of uncertainty with showers moving in and temperature cooling.”
Sunday's sunrise by the River Great Ouse in Ely, Cambridgeshire
Credit: Veronica Johansson Poultney/Bav Media
The TUC urged employers to ensure staff returning to work do not overheat.
"Employers can help workers in hot weather by allowing flexible working, relaxing workplace dress codes and allowing staff to take frequent breaks,” it said.
While there is no maximum temperature limit for workers, government guidance urges employers to ensure that it remains at a comfortable level and that clean and fresh air is provided.
The Chartered Institute of Personnel and Development said research showed around 70 per cent of workers were tempted to call in sick in hot weather.
BrightHR, which monitors 250,000 employees at 10,000 workplaces, said: "The dates lockdown restrictions ease are among the most-requested days off this year, outside major holidays” and the Federation of Small Businesses warned: "Employees’ days off in hot weather do happen and can be a big problem."
The Grantham Research Institute at the London School of Economics has said that productivity drops around eight per cent when temperatures go above 30C, warning: “Heatwaves hit some industries hard."
Beaches across the south coast were packed on Sunday as families made the most of the weather, with car parks at capacity as early as 9am.
But many emergency services were overrun, prompting police forces to urge people not to call 999 unless it was an emergency.
Among them was Surrey Police, which said that locals with non-urgent requests would get a quicker response if they sent a direct message via Twitter.
A woman was airlifted from the beach at Durdle Door in Dorset after fainting following a suspected jellyfish sting.
The coastguard helicopter lands on the beach at Durdle Door in Dorset to deal with a medical emergency
Credit: Graham Hunt/BNPS
In Cumbria, a teenage boy is thought to have died after entering the River Eden, near Stony Holme on Saturday evening. Police confirmed that a body had been found at 11.45am on Sunday.
In Croyde, north Devon, a body was pulled from the sea before crowds of tourists.
HM Coastguard and RNLI crews were called to the scene shortly after 10.30am. Police confirmed a body had been found but said the incident was non-suspicious.
Meanwhile, in south Wales, lifeguards had to carry out a rescue after a dog swam miles out to sea.
After a three-hour search, springer spaniel Ollie was eventually spotted near another beach – three miles from where he initially entered the water in Porthcawl.
RNLI helm Simon Emms said: "He seemed very happy to see us and enjoyed his ride on the lifeboat back to Porthcawl.
"The owner did the right thing by contacting the coastguard.”
Living History historic characters cooling down on the hottest day of the year at Sudeley Castle in Gloucestershire, England
Credit: PAUL NICHOLLS
A young deer was also rescued by a lifeboat crew after being carried three miles across the sea by the tide off the Isle of Wight.
Lifeboat helm Myles Hussey, said: "To say we were surprised was an under-statement. Even the Coastguard had to be told three times before they believed us.”
Several children were treated in hospital for extreme sunburn, prompting experts to warn parents to take the necessary precautions.
Three were admitted to the Welsh Centre for Burns and Plastic Surgery at Swansea’s Morriston hospital throughout Friday and Saturday.
The Swansea Bay University health board said staff feared a further surge in cases in the coming days.
“Please remember to be sun safe,” it said. “Just a few minutes applying a high-factor sun screen could save you and your child a lot of pain and suffering.”
Meanwhile, the RAC warned that this summer will see unprecedented levels of traffic on UK roads due to motorists planning in excess of 29m staycations.
It said Cornwall, Devon, Dorset and Somerset would receive the largest numbers of holidaymakers, followed by Scotland, Yorkshire, the Lake District and East Anglia.