The TUC has organised a nationwide inspection of workplaces this week to check that workers are being helped to cope with the heatwave.
More than 1,000 trade union health and safety reps have signed up to take part.
Working in hot weather can lead to dehydration, tiredness, muscle cramps, fainting, and – in the most extreme cases – loss of consciousness, said the TUC.
Trade union reps will check workplace temperatures, and will discuss with employers the protective steps they can take, including keeping buildings cool, shaded areas for outdoor workers and allowing flexible hours.
TUC general secretary Paul Nowak said: “Everyone should be safe at work, but working in a heatwave can be dangerous – especially when your employer wants you to carry on regardless.
“That’s why union reps are taking action this week to keep their workmates safe in the heat, and it’s why we’re calling on the government to strengthen legal protections with a maximum working temperature.
“A lot of the best safety steps are just common sense.
“For example, employers should allow flexible hours to avoid the hottest parts of the day. And indoor workplaces should be kept at comfortable temperatures, with relaxed dress codes.”