Labour and unions have called for staff to be able to wear casual clothes to work to deal with the heatwave.
With temperatures soaring into the 30s, employers were urged to keep their staff safe from the extreme heat.
The TUC said staff should be allowed to start work earlier, or stay later, leave jackets and ties in the wardrobe and have regular breaks.
General secretary Frances O’Grady said: “We all love the summer sun, but working in sweltering conditions in a baking shop or stifling office can be unbearable and dangerous.
“Indoor workplaces should be kept cool, with relaxed dress codes and flexible working to make use of the coolest hours of the day.
“Bosses must make sure outdoor workers are protected with regular breaks, lots of fluids, plenty of sunscreen and the right protective clothing.”
“Lots of staff are still working from home, so they may struggle to work during the hottest parts of the day. Employers should allow flexible hours so people can work when it’s cooler.”
Andy McDonald, shadow employment rights and protections minister, said: “Following the unprecedented extreme heat warning from the Met Office, the Government must ensure that employers are taking steps to protect their workers.
“Most employers are responsible and want to take steps to protect their staff from extreme heat, but the Government must issue clear guidance.
“Given the dangers of working in heat, there should be a maximum working temperature. Extreme heat is a serious health and safety risk and working people must not be exposed to dangerous conditions that threaten their health and wellbeing.”