Unions have urged employers to be mindful of working conditions following a week of high temperatures.
Tuesday was the hottest day ever recorded in the UK and it is forecast to remain warm next week.
There is no legal maximum working temperature in the UK, but government guidance states employers must ensure the temperature in indoor workplaces is "reasonable".
Frances O'Grady, general secretary of the Trades Union Congress (TUC), said: "Working in sweltering conditions can be unbearable and dangerous.
"There is no law on maximum temperatures. But employers have a duty to keep the temperature at a comfortable level and provide fresh and clean air. And bosses must make sure workers are protected with regular breaks and lots of fluids."
The TUC has long called for a change in the law to require employers to reduce temperatures if they get above 24 degrees Celsius and to provide sun protection and water.
It also wants ministers to introduce a maximum indoor working temperature of 30 degrees Celsius, or 27 degrees Celsius for those doing strenuous jobs, to indicate when work should stop.
Trade union Unite Hospitality said it supported the TUC's campaign and urged employers to take hot weather "seriously".
Hospitality businesses across the UK adjusted trading hours and some temporarily closed this week to keep staff and customers cool.
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