The GMB union says workers need protection as a red warning for extreme heat in parts of England begins.
Lynsey Mann, the GMB's health and safety officer, said: "This hot weather is great for being on a sun lounger, but if you're trying to work through it's no joke.She added: "Ultimately, there needs to be a legal maximum working temperature."
TUC general secretary Frances O'Grady added: "We all love it when the sun comes out, but working in sweltering conditions in a baking shop or stifling office can be unbearable and dangerous." The Unite union has previously said employers have "a legal and moral duty to ensure workers' health is not damaged during the current hot weather."
"If workers show any signs of heat exhaustion, the employer should immediately ensure they stop work and are allowed to recover, without loss of pay."Monday and Tuesday.