Organisers say it is the biggest four-day week pilot to take place anywhere in the world, lasting six months.
Firms taking part will give 100% of workers' pay for 80% of the time, in exchange for a commitment to maintain at least 100% productivity.
The trial is being organised by 4 Day Week Global in partnership with think tank Autonomy, the 4 Day Week UK Campaign, and researchers at Cambridge University, Oxford University and Boston College.
Companies taking part provide products and services ranging from education to workplace consultancy; banking; care; financial services; IT software training; professional development and legal training; housing; automotive supply services; online retail; sustainable homecare; skincare; animation studios; building and construction recruitment services; food and beverage and hospitality; digital marketing; and comprehensive case management services for people recovering from traumatic injury.
Researchers will work with each participating organisation to measure the impact on productivity and the wellbeing of its workers, as well as the impact on the environment and gender equality.
Joe O'Connor, Chief Executive of 4 Day Week Global, said: "The UK is at the crest of a wave of global momentum behind the four-day week.
"As we emerge from the pandemic, more and more companies are recognising that the new frontier for competition is quality of life, and that reduced-hour, output-focused working is the vehicle to give them a competitive edge.
"The impact of the 'great resignation' is now proving that workers from a diverse range of industries can produce better outcomes while working shorter and smarter."
Juliet Schor, Professor of Sociology at Boston College, and lead researcher on the pilot, said: "We'll be analysing how employees respond to having an extra day off, in terms of stress and burnout, job and life satisfaction, health, sleep, energy use, travel and many other aspects of life.
Ed Siegel, chief executive of Charity Bank, which is taking part in the trial, said: "We have long been a champion of flexible working, but the pandemic really moved the goalposts in this regard. For Charity Bank the move to a four-day week seems a natural next step."
"The 20th-century concept of a five-day working week is no longer the best fit for 21st-century business."
"We firmly believe that a four-day week with no change to salary or benefits will create a happier workforce and will have an equally positive impact on business productivity, customer experience and our social mission."