Firms are being urged to relax workplace dress codes to help staff cope with the heatwave.
As well as allowing comfortable clothes, the TUC has suggested that any outside work is done in the morning or afternoon to avoid the searing heat of the midday sun.
The union organisation again called for a change in the law to introduce a maximum indoor temperature, with employers obliged to adopt cooling measures when a workplace temperature reaches 24C (75F).
Companies should supply workers with cool drinks and allow them to take regular breaks, said the TUC.
General secretary Frances O'Grady said: "While many of us will welcome the sunshine and warm temperatures this week, working in sweltering conditions can be unbearable and dangerous.
"Employers can give their staff a break by relaxing dress code rules temporarily and ensuring staff doing outside work are protected.
"Obviously shorts and flip flops won't be the right attire for all workers, but no-one should be made to suffer unnecessarily in the heat for the sake of appearances."