A union has called for maximum workplace temperature guidelines to be created as workers sweat in the continuing hot weather.
While minimum workplace temperatures are clearly specified within health and safety laws, there is no clear legal maximum temperature, although employers are required to make sure that workplace temperature is "reasonable".
Employers may be breaking the law by failing to take action such as providing cool drinking water, letting staff dress down or introducing fans, warns the TUC.
While the TUC recommends a maximum working temperature of 30 C, or 28 C for those doing strenuous work, many employers are confused about what they must do and when they must act due to a lack of legal clarity.
"Bosses who fail to adopt the cool work approach risk damaging their firms' productivity as their employees wilt in the heat," said TUC general secretary Brendan Barber.
"They are also increasing the chances of a member of staff falling ill or having an accident because they were too hot."
Bosses may be liable for illnesses or accidents caused by the heat if they do not act to keep their workplace cool, the union warned.