Employees at 61 companies across Britain worked an average of 34 hours across four days between June and December 2022, while earning their existing salary. Of those, 56 companies, or 92%, opted to continue like that, 18 of them permanently.
The trial covered 2,900 staff in total across different sectors, ranging from finance company Stellar Asset Management to digital manufacturer Rivelin Robotics and a fish-and-chip shop in the coastal town of Wells-next-the-sea.
Employers from the marketing and advertising, professional services and charity sectors were most represented in the trial. Some 66% of those participating had 25 or fewer employees, while 22% had 50 or more staff. 11% were not for profit.
The shorter workweek trial is the largest in the world according to Autonomy, a British-based research organization which published the report alongside a group of academics and with backing from New Zealand-based group 4 Day Week Global.
The trial reflects growing scrutiny of how people work, especially since the COVID-19 pandemic when furlough schemes and mandatory work-from-home periods prompted many to question whether they needed to sit in an office five days a week.
Staff said their well-being and work-life balance had improved while data showed employees were much less likely to quit their jobs as a result of the four-day week policy.
"This is a major breakthrough moment for the movement towards a four-day working week," Joe Ryle, Director of the 4 Day Week Campaign, said in a statement.
Paul Oliver, chief operating officer at Citizens Advice Gateshead, said job retention and recruitment had improved and sickness levels had gone down during the trial.
The majority of participating businesses agreed that productivity had been maintained.
However, while the findings may be interesting for companies struggling for talent, other surveys show very few other British employers plan a four-day week soon.
When the Chartered Institute of Personnel and Development (CIPD), representing human resources professionals, surveyed members last year, it found very few employers expect to move to a four-day week in the next three years.
Two thirds expected no change in the next decade.
However, evidence that it helped to retain staff could prove powerful for companies struggling to recruit workers since the pandemic. Britain has the added complication of its departure from the European Union.
"That should give us a competitive advantage," a senior manager at an insurance firm in the trial said of the evidence of worker retention from a four-day week.