Unions are calling for people to get legal protection against high temperatures in UK workplaces, as a heatwave arrives that could cause temperatures to approach 40C on Monday and Tuesday.

The GMB union said the government should set a maximum temperature for workplaces of 25C, meaning employers should offer flexible working and travel arrangements, give staff extra breaks and relax dress codes to allow staff to wear cooler clothes.

Lynsey Mann, the union’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.

“Bosses need to do everything possible to keep workplaces cool and, more importantly, safe. This can be as simple as letting people wear more casual clothing and providing proper hydration.

“High levels of UV exposure also mean that outdoor workers have a much higher risk of developing skin cancer. Simply allowing more breaks and providing sun cream and protective clothing, such as hats with neck covers, can help reduce this risk.

“Ultimately, there needs to be a legal maximum working temperature in the same way we have a legal minimum working temperature, and it is in employers’ interests – workers who are overheating aren’t going to be at their best.”

Meteorologists have given an 80% chance of the heatwave breaking the UK’s record for highest temperatureof 38.7C, which was reached in 2018.

There is a 50% chance of 40C being broken in an area of the UK that has been given a red weather warning for heat.

While there are rules regarding minimum temperature levels in UK workplaces, there are no laws that set a maximum level.

The Health and Safety Executive (HSE) has previously said that employers have a responsibility to ensure that working conditions are “reasonable”, but there is no specific temperature limit.

The Cabinet Office minister, Kit Malthouse, has said that Monday and Tuesday may be “a moment to work from home” due to travel disruption, while the UK Health Security Agency has warned of risks to health from the soaring heat.

The deputy prime minister, Dominic Raab, said people should stay hydrated, but added that they should be “resilient” and “enjoy the sunshine”.

Some local authorities have warned of disruption to services, and a number of schools in areas covered by the red warning – central, northern, eastern and south-eastern England – have told parents they will remain closed.

The Trades Union Congress general secretary, Frances O’Grady, repeated a call that she made in 2018, amid highs of 35C, for a maximum limit to be set.

She said: “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. Indoor workplaces should be kept cool, with relaxed dress codes and flexible working to make use of the coolest hours of the day.

“Bosses must make sure outdoor workers are protected with regular breaks, lots of fluids, plenty of sunscreen and the right protective clothing.”