The Trades Union Council (TUC) has called for the introduction of a new upper limit on workplace temperature.
Employees are not expected to work when the temperature drops below 16 degrees C – or 13 degrees C for those with physically demanding jobs – but there are no similar restrictions higher up the scale.
The TUC recommends a law change so employers are forced to act when the temperature inside hits 24 degrees C, while staff could be sent home and businesses prosecuted if the mercury climbs to 30 degrees C.
General secretary Brendan Barber said: "No-one is expected to work in sub-zero temperatures, but overheated employees are meant to carry on regardless of how high the office temperature soars. We need to see action now."
According to the organisation, an employer must provide an environment that is practical, safe and without risks to health – which includes the temperature of the workplace.
The Met Office recently forecast summer temperatures across the UK are likely to be warmer than average.