Heatwave health warnings were issued today as the Met Office declared it was the hottest day of the year so far.
In Heathrow, the mercury rose to just under 32C, as Britain sweltered in the hottest spell of weather for seven years.
Meanwhile, police and fire chiefs reiterated warnings about escaping the heat by swimming in open water after four people died at lakes in Norfolk, a river on the Shropshire/North Wales border and the sea in Cornwall.
Public Health England (PHE) officials advised people to stay cool, drink lots of cold fluids and keep an eye those they know to be at risk.
A group of MPs is calling on the government to introduce a rule which would mean workers are sent home when temperatures reach 30C.
David Crausby, MP for Bolton North-East, says a clear maximum temperature is needed to help employers, which could be 27C for those in strenuous jobs and 30C for those in non-strenuous jobs. He says the group of MPs are hoping to hold discussions with employers to take the matter further.
He added: "It's not just about the sun, it's about people who work in occupations where there is heat all the year round, the baking industry for instance and some of the hot metal industries."
Employers are also being urged to allow staff to dress down to make it easier for them to work in the heatwave.
The TUC said firms should also provide fans, portable air conditioning units and cold drinking water to lessen the chances of workers falling ill.
General secretary Frances O'Grady said any prolonged hot and sunny spell was not going to be fun if workers were "trapped" inside an overheated workplace.