As the UK heatwave continues, unions have warned bosses they may be breaking the law if they fail to protect workers from the effects of extreme heat.
The heatwave was set to continue in parts of the UK
TUC, the union body, said if accidents happen due to staff feeling ill or faint because of heat in the workplace, employers could be liable.
The TUC says regulations oblige firms to keep temperatures "reasonable".
It has called for a maximum working temperature of 30C or 27C for people doing strenuous work.
The TUC says the lack of a clear maximum workplace temperature under UK law means many employers are confused about what they must do and when they should act.
TUC General Secretary Brendan Barber said: "Whilst most sensible employers will be doing their utmost to keep their staff cool in the oppressive heat, there will always be some too mean to do anything to bring their office or shop temperatures down.
"Bosses who fail to adopt the 'cool work' approach risk damaging their firms' productivity as their employees wilt in the heat.
"They also increase their chances of ending up in court, because someone has fallen ill or has had an accident because it got too hot."
The TUC says there is a lot employers can do to keep staff cool, and last week it launched its "cool work campaign".
It cites health and safety regulations saying employers should reduce heat by introducing air conditioning, distributing fans, providing lots of cold drinking water and moving staff away from windows.
Mr Barber added that "a clear maximum temperature would make a real difference".
However, the Health and Safety Executive said that setting a maximum working temperature was "inappropriate" and would be difficult to enforce.
An HSE spokesman said: "There are almost four million workplaces and one solution will not fit all.
"Employers are required to undertake a risk assessment in their workplace to include thermal comfort."
Help for employees is provided in the HSE's Thermal Comfort in the Workplace guidance, he added.
Meanwhile, the Met Office has again issued a heatwave health warning - indicating there was 100% probability of heatwave conditions in parts of England and Wales - for between 1000BST Wednesday and 0800BST on Thursday.
In London and the South East the probability of such conditions was set at 100%, while the risk was only 20% in Wales, north-west and south-west England.
Meanwhile, eastern Scotland and northern Isles saw rain on Wednesday, and forecasters said storms were expected to hit areas of the South and eastern England on Thursday.