The TUC said travelling to work, and being in hot offices, would be uncomfortable because of predicted temperatures as high as 32C tomorrow.
Workers should be allowed to wear shorts or vest tops rather than jackets and ties, and should be encouraged to take regular breaks, it was argued.
The union organisation pointed out that while there is a legal limit for working in cold temperatures (16C), there is no upper limit.
Bosses who provide cool and comfortable work environments will get more out of their staff when it’s sweltering, said the TUC, while those who are unable to dress in cool summer clothing and who work where there is no air-conditioning will feel lethargic.
TUC general secretary Frances O’Grady said: “It’s no fun working in a baking office or factory and employers should do all they can to take the temperature down. Clearly vest tops and shorts are not suitable attire for all front line staff, but those not dealing with the public should be able to discard their tights, ties and suits.
“Extreme heat can be as unpleasant to work in as extreme cold, and so long as the UK has no legal maximum working temperature, many workers will be working in conditions that are not just personally unpleasant, but will also be affecting their productivity.
“With temperatures set to soar this week, now is the time for employers to relax the dress code rules temporarily and allow their staff to dress down. Making sure that everyone has access to fans, portable air conditioning units and cold drinking water should help reduce the heat in offices, factories, shops, hospitals, schools and other workplaces across the country.”
The Chartered Management Institute said many businesses underestimated the risk posed by weather issues.
Productivity can drop as temperatures soar, and some employees may be tempted to take an extra day’s leave, said the CMI.
“It is therefore vital for employers to ensure they have business continuity plans in place to prepare for these situations, to ensure that productivity doesn’t drop and that flexible working arrangements are on offer, should employees require these.
“The heatwave is likely to be a major cause of disruption to UK businesses and will impact employees travelling to and from work, as well as during the working day,” said a spokesman.