Office staff should be sent home when the temperature rises above 30C, while manual workers should be able to down tools when the mercury tops 27C.
The Trades Union Congress wants employees to be allowed to leave work when to becomes too hot - with values expected to hit record summer highs in parts of the UK this week.
Employers have been advised to 'adopt cooling measures' when the workplace temperature hits 24C.
At the minute, there isn't actually a legally defined maximum or minimum temperature for the workplace.
Guidelines issued by the Health and Safety Executive, however, state that the temperatures inside workplace buildings should be 'reasonable' .
The TUC says bosses should allow workers to adopt flexible working.
This would mean staff can come in earlier or leave later to avoid hot conditions in the rush-hour commute. It also encourages employees to relax workplace dress codes and keep buildings cool, the Manchester Evening News reports.
TUC General Secretary Frances O'Grady said: "While many of us love to see the sun, it's no fun working in a baking office or a stifling factory.
"Bosses should do all they can to keep the temperature down.
"The easiest way for staff to keep cool inside is being able to work in more casual clothing.
"While shorts and vest tops may not be appropriate for all, nobody should be made to suffer in the heat for the sake of keeping up appearances.
"It's in bosses' interests to provide a cool and comfortable work environment.
"Workers who are unable to dress down in lighter clothing, or who work in offices without air-conditioning, fans or drinking water, are going to be tired, and lack inspiration and creativity."
