Five days on, two days off has been the defining pulse of British labour for more than 80 years. But as 70 UK companies embark on the largest trial yet of a four-day week, the working calendar may finally be changing.

Campaigners are seizing on the way Covid shook up working lives to push back the boundaries of the weekend for the first time since the postwar years when the whole of Saturday became a day off for most. One advocate predicts a four-day week could be available to the majority in Britain within five years, and Stephen Fry this week gave his voice to an increasingly confident four-day week campaign, which argues shorter hours boosts productivity, cuts carbon emissions and improves family life – all without cutting pay.

In the campaign, Fry suggests the seven-day week should no longer be considered “a brute fact” because it is “not real the way a day is real, a single spin of our planet, or the way a year is real, one lap of the Earth round the sun … the week was invented by us.”

Ancient Egyptians had a 10-day week, the Romans operated an eight-day week for a while, and in the 1920s the Soviet Union experimented with a five-day week with rest days staggered for different parts of the workforce to keep the machines ticking nonstop.

A brewery, a fish and chip shop, an inheritance tax specialist and a software firm are among firms taking part in the six-month trial that will be monitored by academics at the universities of Oxford and Cambridge and Boston College in the US.

But if the idea that Friday is the new Saturday sounds too good to be true, it may be. It has some heavyweight critics, especially as Britain was this week forecast to plunge into negative economic growth next year.

Robert Skidelsky, an economist who examined the idea of a national four-day week for the former shadow chancellor John McDonnell, said: “It can’t be done on a legislative national basis.” He said that with real incomes about to fall in the face of rising food and energy prices, people would want to work more, not less, to maintain their standard of living.

Skidelsky advocates reducing working hours where the benefits of automation can be accrued, but cautioned that a shorter working week on the same pay – the principle of the four-day week campaign – is out of reach for “people in the gig economy who have to scrape and do several jobs to keep up a standard of living”.

One delivery courier in Peterborough paid on a per-parcel rate that hasn’t changed in the face of inflation, and now facing soaring costs as petrol hits £2 a litre, responded to news of the drive with scepticism: “We won’t benefit, will we? Couriers are panicking and need to work more.”

A drive for more, not less, work is also being seen abroad. In China, companies are squeezing in more hours through the brutal-sounding 996 system – 9am to 9pm, six days a week. Elon Musk is the poster boy for entrepreneurs to “work as hard as hell” with 100-hour weeks.

Nevertheless, in the UK, North America and across Europe a more relaxing four-day week is becoming reality for some. The Spanish city of Valencia this month announced subsidies for employers to trial the shift, the electronics giant Panasonic in January announced an optional shorter week in Japan, and up to 50 firms are expected to join a four-day-week pilot in Australia.

“I think this is unstoppable,” said Andrew Barnes, who founded the four-day week global campaign after seeing a trial in New Zealand increase worker productivity to 125% of previous levels and sick days halved. “We are using a method of working designed for the repetitive manufacturing industry in the 1920s and we are applying that to the 21st century. It makes no sense.”

He predicted the majority of employers in the UK could be offering shorter hours in five years.

Lorraine Gray, the chief executive of Pursuit Marketing, a call centre business in Glasgow with clients including the NHS and Google, has 350 members of staff who work only Monday to Thursday. She said since the company first trialled the switch in 2016, productivity had increased by 29% and now only 12% of staff left each year, compared with 17% previously.

She said call centres often suffered from “Monday-itis” – workers calling in sick to get a long weekend. After switching to the four-day week, sickness dropped to almost zero. They found that many sick days were taken by staff only needing a couple of hours off for an appointment, but with the Friday now available for “personal admin” that had largely stopped.

“We get mums who say they now have time when the kids are at school so they can get their hair done or do housework and then the weekend is just quality time,” she said. “Some people do Open University courses, others look after elderly relations.”

Proponents of the shorter working week say it is possible because so much time is wasted at work because of distractions. When Barnes ran Perpetual Guardian in New Zealand, where he trialled the four-day pattern, he found workers’ use of non-business related websites fell 35%.

But squeezing the same amount of work into fewer hours may only be possible for people who manage their own workload; it is harder to boost productivity in occupations such as hospitality and public services such as nursing and policing.

The Vault City brewery in Edinburgh has been running a four-day week from Monday to Thursday since January on the same pay. Richard Wardrop, 32, the head of marketing, said he was attracted to join by the short week and he would “struggle now with a five-day week”. He has been using the time to travel, with long weekends in places such as Aviemore, and enjoys shopping and going out on a Friday when it’s quiet “before the chaos of the weekend”.

Anecdotally, productivity is up, but the company plans to add weekend shifts because the yeast that makes the fermentation happen rests for no one. When it comes to the four-day week, “the beer doesn’t always play ball”, Wardrop said.

Some advocates of the four-day week have dreams beyond having more time to get a boiler fixed or go shopping. Alex Soojung-Kim Pang, the author of Rest, a book subtitled “Why you get more done when you work less”, makes the point that a four-day week creates an entire year of extra free time every five years.

“Imagine what you would do with another year. Now imagine corporate strategies and government policies that create millions of years of free time by shortening the workweek. How much better could the world be if we had time to fix it?”