Employers should allow staff to work flexibly and have a relaxed dress code during this week's UK heatwave, the Trades Union Congress (TUC) has said.
Temperatures are set to top 35C and TUC general secretary Frances O'Grady said: "It's in bosses' interests to provide a cool and comfortable work environment."
The TUC wants people to be able to travel at different times or work from home and wear more casual clothing.
It adds staff should have breaks often and cold drinks should be available.
Ms O'Grady added: "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. Nobody should be made to suffer in the heat for the sake of keeping up appearances.
"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."
There are no laws in the UK about when it is too hot to work, but the TUC would like that to change.
It wants the introduction of a new maximum indoor temperature, set at 30 degrees - or 27 degrees for those doing strenuous jobs - with employers obliged to adopt cooling measures when the workplace temperature hits 24 degrees.